CHAPTER X_The age of the Earth

RADIOACTIVE MINERALS AND THEIR AGES

The rate of helium production from uranium and thorium—Lead

the final product of the uranium family—Its accumulation

in geological time—Lead and helium ratios as a measure

of geological time—Assumptions to be granted—Value of

analyses in deciding quality of material for estimating

ratios—Necessity for fresh, stable, primary rock

minerals—Strutt’s work on the helium ratio—The lead

ratio—Boltwood’s collection of analyses—Examples, with

geological data—The author’s work on the Devonian minerals

of Norway.

As we saw in Chapter VII the α-particles which are emitted at certain

points in the line of descent of the radioactive elements have been

identified with helium. Fortunately the evidence is conclusive, for

upon this identification depends the latest and most elegant method yet

devised of measuring geological time.

When all the members of a genetically related series of radio-elements

are in equilibrium, the transformation proceeds in such a way that an

equal number of atoms of each element disintegrates in the same time.

When an atom of uranium disintegrates it does so with the production

of two atoms of helium. At the same time, as indicated in the diagram,

page 190, each of six other members of the family also emit a single

atom of helium. Consequently, the total number of helium atoms which

are liberated in the course of the complete transformation of a

single atom of uranium is eight. From this result and the remarkable

measurement made by Rutherford and Geiger—that 3·4 × 10¹⁰ α-particles

or atoms of helium are expelled per second from a gram of radium—it

is possible to calculate that the annual production of helium from

a gram of uranium in equilibrium with all the other products of the

series is 10·7 × 10⁻⁸ cubic centimetres. Measurements of the ionising

power of thorium and its chain of dependent elements may be utilised to

calculate the same rate in the case of the thorium family. It is found

that one gram of thorium is equivalent in helium generation to 0·26

gram of uranium. The details of these calculations will be found in

Appendix A.

While there is little uncertainty in these indirect results, it is

gratifying to know that Prof. Strutt, during 1909-10, verified them

both by a direct appeal to experiment. In certain minerals which

contain radioactive constituents, the evolution and accumulation of

helium must have been steadily proceeding for very long periods. Before

a direct determination of the rate of helium production can be made, it

is evidently necessary to use material entirely free from that element.

Strutt worked with richly radioactive minerals, from large quantities

of which he expelled completely the accumulated store of helium. This

was done by preparing solutions with every precaution to avoid the

presence of undissolved particles of mineral, and afterwards boiling

them till the helium was removed. The solutions were then put aside

until a fresh supply of helium had been generated in sufficient

quantity to be detected and measured. To isolate the gas from the

solutions and accurately to determine its volume was obviously a matter

of great experimental difficulty, so minute were the volumes dealt

with. However, as the result of the exquisite delicacy of his methods,

Strutt brought his experiments to a successful issue.

The minerals used were pitchblende and thorianite, the former

containing uranium alone, and the latter both uranium and thorium.

The annual production of helium per gram of the parent element (in

radioactive equilibrium with its respective family) was found to be:

(_a_) in the case of uranium:

10·6 × 10⁻⁸ ccs. or 1·88 × 10⁻¹¹ grams.

10·7 × 10⁻⁸ ccs. was the calculated estimate.

1 cc. would therefore be formed in 9,600,000 years.

(_b_) in the case of thorium: 2·4 × 10⁻⁸ ccs.

1 gram of thorium is therefore equivalent in its

rate of helium generation to 0·23 grams of uranium,

the calculated estimate being 0·26 grams.

The remarkable concordance of these results with the theoretical

requirements is an eloquent tribute to the refined methods and

experimental skill with which the measurements were carried out.

Experiments carried out by Boltwood and Rutherford in 1911 afford

an equally striking confirmation of the conclusions on which the

theoretical results were based. They measured the rate of production

of helium from radium, the latter being in equilibrium with its

early disintegration products, three of which also emit α-particles.

Radio-lead and polonium were completely removed. The annual evolution

of helium to be expected was 158 cubic millimetres. The first direct

determinations were made by Sir James Dewar in 1908, and his best

results corresponded to 169 cubic millimetres. Boltwood and Rutherford

arrived at a much closer agreement, their figure being 156 cubic

millimetres.

These results cannot fail to inspire the conviction that our atomic

theory of matter is essentially correct. We are in possession of two

experimental facts. The number of helium atoms expelled per second from

a gram of radium has been directly counted, and the volume of helium

accumulated in a year has been directly measured. The number of atoms

in a given volume of helium (at N.P.T.) can be deduced at once, and the

calculation is independent of any underlying theory. Calculation gives

2·69 × 10¹⁹ atoms per cubic centimetre; the atomic theory demands 2·72

× 10¹⁹.

Although there can now be no doubt that helium is one of the stable

disintegration products, yet there is no direct evidence as to the

identity of the ultimate products in the direct line of descent. In the

uranium series indirect evidence points to lead with a considerable

degree of certainty, but the end product of the thorium family is still

unrecognised. In every uranium-bearing mineral the parent element

slowly breaks down, while the final product of the transformation

accumulates at its expense. Hence, if lead is the favoured element it

ought to be found in association with uranium in all minerals which

contain the latter. Moreover, in minerals which can be proved to be

of the same antiquity, the amount of lead per gram of uranium should

be constant; further, in minerals of various geological ages the

proportion of lead should vary according to the latter. A mineral which

began its accumulation of lead in pre-Cambrian times should certainly

contain more at the present time than one in which lead has been

collecting only since, say, the Tertiary outburst of igneous activity.

The same statements apply equally well to the case of helium.

In so far as these principles may be used conversely to test the

identity of lead with the ultimate product, they lend every support to

that important conclusion. Dr. Hillebrand, the leading authority on the

analysis of uranium-bearing minerals, has never in the course of a long

experience found uranium unaccompanied by lead. It was this constant

association which led Boltwood, in 1905, to suggest the probability of

a genetic relationship existing between these two elements. In 1907,

Boltwood went farther and showed that for minerals of the same age the

amount of lead for each gram of uranium, or the ratio Pb/U, was, in

general, nearly constant. He collected all the best analyses of primary

uranium minerals, but unfortunately he omitted to give the geological

details of their occurrence. As will be seen in the present chapter,

when the relative ages of the minerals are compared with their lead

ratios, a striking proportionality discloses itself.

The evidence of atomic weights is also favourable to lead. The complete

disintegration of an original atom of uranium may be expressed as

follows:

U ➜ 3He + Ra ➜ 8He + Pb

238·5 3·994 226·36 3·994 207·08

———————————— —————————————

238·5 238·34 239·03

Atomic weights are appended to each symbol, and the totals, which

should be equal, are placed underneath. The agreement is close, but

not as convincing as one could desire. Two alternative explanations of

the discrepancies are suggested. Either lead is not the final product,

or the atomic weights of both radium and uranium are too low by about

0·5. Neither alternative can readily be granted, but it may be pointed

out that since uranium is the heaviest known element and radium follows

not far behind, any impurities whatever, with the exception of thorium,

would have the effect of lowering the observed atomic weights.

Accepting the above equation as substantially correct, the mass of

lead generated in one year from a gram of uranium can now easily be

calculated. For eight atoms of helium, one of lead is produced, or,

mass for mass, six and a half times as much.

Consequently, as a gram of uranium involves the annual production of

1·88 × 10⁻¹¹ grams of helium, the associated lead which remains must

amount to 1·22 = 10⁻¹⁰ grams. If this rate were constant we could find

how long it would take for any mass of uranium to become completely

converted into helium and lead. However, the rate is not constant, but

is proportional at every moment to the quantity of uranium remaining

unchanged. As the parent element becomes exhausted, it disintegrates

more and more slowly.

Now in very considerable periods amounting to hundreds of millions of

years only a very small fraction of the uranium originally in existence

is decayed. For this reason, if only a small proportion of lead or

helium has collected in a mineral since it began its life-history, then

no serious error will be made in assuming their rates of evolution to

have been constant. In minerals which have been in existence for 400

million years the slowing down is only about 5%. If an appreciable

error should arise in ignoring this decline, then in place of the

present day percentage of uranium the time-average must be substituted

(see Ap. A, p. 179); that is to say, the amount of uranium which, if it

did break up at a regular rate, would evolve the same quantities of the

ultimate products.

Having calculated this uranium average, Uₘ, with the necessary

approximation, the total quantity of lead or helium accumulated in a

mineral would then provide a direct measure of its age.

Using lead as the age-index, and knowing its percentage, Pbₜ, in the

mineral, then the time it has taken to collect, i.e. the age of the

mineral, Pbₜ, is given by Pbₜ/Uₘ × 8200 million years.

Using helium as the age-index, both thorium and uranium must be

estimated. The amount of thorium may be conveniently expressed in terms

of uranium, since the latter is four times as active in its helium

production as thorium. The total equivalent quantity of uranium, Uₑ,

is thus known, and thorium then need play no further part in the

calculations. It is found most convenient to measure the amount of

helium, Heₜ, as the volume in cubic centimetres per gram of mineral. In

this notation the age is given by Heₜ/Uₑ × 9·6 million years.

The validity of this procedure evidently demands the granting of

certain obvious assumptions. Our choice of suitable minerals will

not only be limited by these considerations, but the reason for a

particular choice will be justified. The assumptions fall under the

following headings:

(_a_) That no appreciable amount of lead or helium was present

at the genesis of the mineral.

(_b_) That no lead, helium, or uranium has subsequently been

added or removed by external agencies.

(_c_) That no lead or helium has originated by any other

radioactive process than those already suggested.

The first and second suppositions bring up the whole problem of the

origin of minerals. It is possible, by means of a physical examination,

to decide whether a mineral is of the same age as the rock in which it

occurs, or whether it is older or younger. In igneous rocks, such as a

granite, the majority of the minerals are of the same antiquity as the

rock, that is, they date from the period of consolidation of the rock

magma. The component mineral particles of most sedimentary and detrital

rocks existed long before the strata were laid down, whereas the

cementing materials by which they are consolidated are partly furnished

by percolating solutions and therefore may be subsequent to the period

of deposition. In the same category come those ore deposits which

occupy the fissures and crevices of pre-existing formations.

In whatever way a mineral may occur, its history can always be

traced back either directly or by conjecture to an igneous rock, and

it is rarely that it is possible to go beyond the magma from which

such a rock must have consolidated. And even if this can be done in

exceptional cases, there lies behind still another magma to which the

material can be referred. Consequently, the minerals of igneous rocks

are regarded as primary or original, and here we may briefly consider a

few facts relative to their crystallisation from a molten condition.

A molten rock is regarded as a solution in which the numerous

constituents are dissolved one in another. Now certain of the

constituents can only remain in solution provided the latter is, with

regard to them, very dilute. That is to say, they are only slightly

soluble in a solvent composed of the rest of the rock material.

Consequently, substances of which these constituents form an essential

part will, as a general rule, be the first to crystallise; as examples,

zircon, sphene, and apatite may be cited. For the same reason it

happens that the magma does not remain homogeneous, but rejects certain

of the rarer elements which collect together in a subsidiary magma of

peculiar composition. To this concentrate of exceptional constituents,

the gases and water vapour expelled during solidification are also

added, and serve to maintain it in a state of æquo-igneous fusion,

even when the bulk of the rock has already crystallised. The residual

liquors yield the minerals of pegmatites and of drusy cavities. Certain

minerals which are conspicuously rare in the body of the normal rock,

are often developed on a large scale in pegmatite dykes, and it is from

these that the most perfect and beautiful crystal forms are generally

obtained.

Amongst the elements of limited solubility in a rock magma, uranium

and thorium must be placed. The accessory minerals of ordinary

igneous rocks, such as those already mentioned—zircon, sphene, and

apatite—are rich in the radioactive elements when compared with

commoner minerals like felspar and hornblende. In general, the richness

of a mineral in uranium seems to depend on its position in the order of

consolidation. The minerals first to be formed claim the greater part

of the available store. The original surplus, unable to dissolve in the

magma, is held over, and, should it be rich in radioactive ingredients,

uranium- and thorium-bearing minerals may be formed during the later

stage of pegmatitic intrusions. It is a striking fact that, as primary

constituents, these minerals invariably occur in pegmatites associated

with granite or syenite. As examples, pitchblende or uraninite,

thorite, thorianite, and monazite may be mentioned.

We must now consider the part played by lead and helium during the

genesis of minerals. Before the consolidation of the magma, both

these elements must, of course, have been generated within it for

an unknown period. As to the effect of physical conditions upon

radioactive transformations it has already been shown (p. 102) that

all the evidence points to the conclusion that these atomic changes

are independent of the temperatures and pressures under which a molten

magma exists. The helium already present at the time of crystallisation

appears to behave physically in no way different from the other gases.

There is no evidence that it tends to congregate in any particular

mineral. A small proportion may be distributed through the resulting

rock, but probably the larger share is expelled.

The lead which may be originally present follows a similar course.

The metal is rejected, not only by the primary magma but, with rare

exceptions, by the residual magma also. It finds no definite place

in igneous rocks. Doubtless a certain amount of lead is retained in

the molecular network of crystals, but that amount is not high. In

the rocks of Leadville, Colorado, Hillebrand found an average of less

than 0·002 per cent of lead. In the nepheline syenite of Southern

Norway, using specimens free from minerals which one would expect to

be comparatively rich in accumulated lead, the present writer was able

to determine a percentage of only 0·0004. If, then, there should be

initially a greater quantity of original lead, where are we to look for

it? Probably the most of it goes to form lead-ores, such as galena.

Separated from the pegmatites it appears in the later phases of

ore deposition which follow on the heels of igneous activity. In

company with hot gases, sulphide solutions, and a number of metallic

companions, our lead is carried away and deposited in the fissures

encountered by the mineralised waters. If the agency of magmatic gases

appears to have been an important factor in the production of ore

bodies, the origin of the latter is said to be pneumatolytic. Brögger

has shown that in Southern Norway galena was one of the minerals to be

formed in this way.

In the last phase of this complex series of operations, magmatic waters

contribute their share to the filling of mineral veins, and it is

amongst these hydatogenetic ores that galena is most usually found.

The important point is that lead, for the most part, is drawn from the

primary magma at, or perhaps before, the time of crystallisation, and

it is not until the igneous activities have declined that it again

appears in an active rôle.

Let us now consider the effect of the original distribution of lead and

helium in a newly-formed rock. It will be clear that an analysis of

the rock as a whole would give values of Pb/U and of He/U much higher

than those corresponding to the period since consolidation. Of the

total amounts of lead and helium, part would be originally segregated

in the rock and part would be due to subsequent genesis. In most rocks,

the former part is of sufficient magnitude altogether to invalidate

the use of the ratios as age indices. This difficulty can be avoided

by confining attention to particular minerals—indeed, to just those

minerals which concentrate within themselves the radioactive parent

elements. Within them lead and helium may accumulate to such a degree

that the amount initially present becomes negligible. Zircon from the

Devonian syenites of Southern Norway contains more than twenty times

as much lead as the rock in which it occurs. Roughly, we may say that

since the zircon came into being its content of lead has multiplied

twenty times. Thorite may accumulate a hundred or a thousand times as

much lead as it possessed at first. For the same reason, minerals like

zircon and sphene often contain hundreds of times as much helium as

the rock from which they are taken, and there is little possibility

of error in assuming that they have themselves generated the whole of

their supply.

Another difficulty, and a more serious one, must now be faced. Can we

be sure that for periods of hundreds of thousands years a mineral has

remained comparatively unaltered by external agencies? With regard

to helium there is undoubtedly a tendency to escape. Strutt has

demonstrated that when a radioactive mineral has been powdered, helium

begins to leak, rapidly at first, then at a diminishing rate. Even

crystals washed out of their original matrix showed a considerable

leakage of helium. The observed rate of escape always exceeds the rate

of generation, and it therefore follows that during the life-history of

a mineral the conditions must be specially favourable to the retention

of helium, for otherwise the latter could not have accumulated.

Nevertheless, these experiments prove conclusively that the majority of

minerals do not contain their full store of helium, and it is a matter

for surprise that they contain so much. Consequently, ages deduced from

the helium content of minerals can be regarded only as a fraction of

the true age.

Dealing with lead and uranium, we must consider the tendency to

alteration of the minerals in which they occur. From the surface down

to the permanent level of the ground waters, rock material is subject

to weathering. The more soluble constituents are leached out and

complex silicate minerals are decomposed by the combined action of

water, oxygen, and carbon-dioxide. It is in this belt of weathering

that igneous rocks suffer most change. Many of the primary minerals

are broken down and alteration products take their place. All the

reactions involve considerable increase in volume, and not only are

minerals altered in place, but material is carried away and deposited

elsewhere as secondary minerals. Can we be sure that lead and uranium

have remained untouched during this redistribution? In some cases we

cannot, but fortunately for our purpose many of the most valuable

minerals, like zircon, are dense and exceptionally stable. A mineral is

only stable over a limited range of conditions. It adapts itself more

or less readily to its physical environment. Certain minerals, however,

are much more capable than others of withstanding great changes without

undergoing metamorphism or alteration, and amongst these are many of

the uranium-bearing minerals.

When there has been a migration of lead or uranium, an appeal to

analysis will rarely fail to dispel the difficulty by disclosing the

fact. It is inconsistent with the chemical properties of these elements

that both should have been affected in the same proportion, and hence

the ratio of lead to uranium obtained from different minerals of the

same geological age affords an immediate test of the extent to which

they have suffered from alteration in the course of their history.

If the analyses give consistent results, it can be safely assumed

that the effects of alteration have been inconsiderable; if there are

marked discrepancies the results must be rejected as valueless from

a chronological point of view. A microscopical examination of the

minerals before analysis is a useful safeguard, for in this way altered

material can often be detected. It is clear that reliable conclusions

can only be drawn from minerals which are undoubtedly _fresh_.

Becker has criticised the method by directing attention to a suite of

minerals from Llano Co., Texas. Their geological age is well defined.

The Burnet granites with which they are associated are intrusive into

a series of schists and quartzites, metamorphosed sediments of late

Algonkian time. The Cambrian rocks lie upon this complex and the period

of intrusion is therefore between two limits which are not very far

apart. The lead ratios of these minerals are far from being constant,

as the following examples show:

Yttrialite 1·15

Yttrialite 0·51

Mackintoshite 0·39

Uraninite 0·17

Fergusonite 1·04

Fergusonite 0·30

Boltwood found a satisfactory agreement in four cases, the ratio being

0·17, but, as he pointed out himself, most of the minerals from this

locality are unsuitable, because of incipient or advanced alteration.

The quartzose pegmatites in which the minerals occur are riddled with

alteration products and secondary minerals, and the whole series is

altogether unfavourable to accurate age determination. It is doubtful

whether the apparent agreement of the ratios quoted by Boltwood ought

to be accepted without further verification; for the present they

cannot be regarded without suspicion.

This example shows how the actual results indicate the vicissitudes,

varying from mineral to mineral, which the lead and uranium contents

may have undergone. The method confirms or denies the validity of its

application in every case. Judging from the relative solubilities of

the constituents in question, uranium is likely to be abstracted from

a mineral during the process of weathering more readily than lead,

and consequently the age deduced from a weathered or altered specimen

should in general be too high. A differential effect of this kind would

account for the high ratios given by the Llano Co. minerals.

Strict attention must be paid to the question of origin, and secondary

minerals avoided as carefully as altered primary minerals. Pitchblende

is often secondary, e.g. when it occurs in veins with metalliferous

sulphides. Other examples, of a rather different type, are autunite

and carnotite. Secondary minerals are necessarily more recent than the

rocks in which they occur, and many of them date back to no very remote

period. Autunite is sometimes formed quite near the surface, within

a few inches in fact. Its antiquity cannot therefore be more than a

few thousand years, and in this time a detectable quantity of lead

could not be generated. The traces actually found were probably in the

original possession of the mineral. In an analysis made by the writer,

only 0·06% of lead was found in specimens of autunite from Mozambique,

where it occurs in bright green flakes attached to the large biotite

crystals of pegmatitic dykes. In keeping with the age of the pegmatites

and the high proportion of uranium—45%—it should have contained a

hundred times as much had it been a primary mineral. The paucity of

lead in autunite has even been put forward as an argument against the

contention that lead is the ultimate product of disintegration of the

uranium family. We now see how baseless is this argument when the

origin of the mineral is remembered. It is not surprising that autunite

should contain so little lead; on the contrary, it contains much more

than the uranium can account for in the time at its disposal.

From these considerations it will be obvious that the only minerals

to be chosen as material from which to determine the lead-ratio are

fresh, stable primary rock-minerals. Having decided this, there remains

a third possibility which might cast doubt upon the method. It can be

objected that lead may originate as a product of some element other

than uranium. Analytical results show clearly that thorium cannot give

rise to lead, or a more proportionate relationship between these two

elements would have announced the fact. There is also a possibility

that certain of the longer-lived members of the uranium family may

themselves be segregated in a mineral, independently of uranium. If

so, they would gradually disintegrate, leaving no trace of themselves

other than the residual helium and lead. In a magma containing 10%

uranium, the radium would amount only to 0·0000034%. In actual magmas,

even of pegmatites, the quantity present is always much less than

this, and even if it be allowed that such tiny quantities may saturate

the magmatic solution, the precipitation and concentration in any

particular mineral would not be sufficient to leave an appreciable

residue of lead.

The application of the accumulation of helium in minerals to the

measurement of geological time, was first suggested by Rutherford in

1905, when he wrote: “I think that, when the constants required for

these calculations are more definitely fixed, this method will probably

give fairly trustworthy information as to the probable age of some of

the radioactive minerals of the earth’s crust, and indirectly as to the

age of the rocks in which they are found.”

During the years 1908-10, Strutt examined a great number of minerals,

and determined the helium ratio whenever practicable. His first set

of experiments dealt with phosphatic nodules and phosphatised bones.

These may sometimes contain fifty times as much uranium as average rock

material, and they have a further advantage in that they can be found

in strata of nearly every age. As they frequently consist of fossils

characteristic of the formations in which they occur, their age is

well defined. However, the power of retaining helium is both poor

and variable in the case of these phosphates, and the time relation

is therefore obscured. Such materials never retain more than a small

fraction of the helium which has been generated within them.

More suitable in their power of retention are certain iron ores,

from which significant results were obtained. The helium ratio, and

therefore the numerical age derived from it, showed a marked dependency

upon the geological age of the mineral, as the following examples

illustrate:

---------+---------------------+--------------

MINERAL. | GEOLOGICAL AGE. | MILLIONS OF

| | YEARS.

---------+---------------------+--------------

Siderite | Upper Oligocene | 8·4

Hæmatite | Eocene | 30·8

Hæmatite | Upper Carboniferous | 141·9

Hæmatite | Devonian | 145·2

---------+---------------------+--------------

Strutt next investigated the more compact minerals of igneous rocks,

notably zircon and sphene. Zircon can be obtained from rocks belonging

to several periods of igneous activity, and being a durable and stable

mineral it is peculiarly fitted to retain the helium generated within

it. Even allowing that the helium found does not represent the whole

amount generated, it is unlikely that the fraction lost will vary as

conspicuously as in the case of phosphates. In so far as that fraction

depends on the structure of the mineral it is probably more uniform for

zircon than for most other minerals. The helium ratio ought therefore

to stand in a close relation to the geological age of the specimen.

That it does so is clearly demonstrated by Strutt’s results, which are

given below. The geological ages have been taken from the most recent

literature, and are given in greater detail than those published in the

original paper. Where two periods are bracketed together they are to

be understood as referring to the limits between which the age of the

igneous rock may fall.

-------------------------+---------------------+--------------

LOCALITY. | GEOLOGICAL AGE. | MILLIONS

| | OF YEARS.

-------------------------+---------------------+--------------

Mt. Somma, Vesuvius | { Recent |

| { Pleistocene | 0·1

| |

Mayen, Eifel | Pleistocene | 1·0

| |

Campbell I., N.Z. | Pliocene | 2·5

| |

Expailly, Auvergne | Miocene | 6·3

| |

Brevig, Norway | Devonian | 54

| |

Cheyenne Canon, | { Upper Cambrian |

Colorado | { Archean | 141

| |

Green River, | { Carboniferous |

N. Carolina | { Archean | 147

| |

Ural Mts. | Pre-Devonian | 209

| |

Ceylon | Archean | 286

| |

_Blue Ground_, Kimberley | Archean | 321

| |

Sebastopol, Ontario | Archean | 622

-------------------------+---------------------+--------------

The results for sphene add but little to the above table, for most of

the rocks from which workable quantities of sphene can be obtained are

of pre-Cambrian age. The most notable helium ratio was from a specimen

occurring in the Archean rocks of Ontario, and corresponded to an age

of 715 million years. Summarising all the data afforded by Strutt’s

work, we may graduate the geological column with a time-scale. It must

be clearly understood, however, that the ages as expressed in years

are, in the case of the helium ratio, minimum values only. How much

greater the time represented by the geological periods actually is will

appear from the ages as deduced from the lead-ratio. A few of these are

placed in the table below for comparison.

-------------------------+-----------------------------------

| TIME-SCALE IN MILLIONS OF YEARS.

THE GEOLOGICAL SYSTEMS. +------------------+----------------

| HELIUM RATIO. | LEAD RATIO.

-------------------------+------------------+----------------

Pleistocene | 1 | --

Pliocene | 2·5 | --

Miocene | 6·3 | --

Oligocene | 8·4 | --

Eocene | 30·8 | --

Cretaceous | -- | --

Jurassic | -- | --

Triassic | -- | --

Permian | -- | --

Carboniferous | 146 | 340

Devonian | 145 | 370

Silurian | } | }

Ordivician | } | } 430

Cambrian | } 209 | --

Algonkian | } | 1000-1200

Archean | 710 | 1400-1600

-------------------------+------------------+----------------

The application of the lead-ratio to the measurement of the antiquity

of minerals was first due to Boltwood. He tested its reliability by an

appeal to the best analyses published up to 1907. Some of these, with

geological details, will now be given.

In Glastonbury, and also in Portland, Connecticut, primary uraninite

is found in the felspar quarries. The pegmatite in which the mineral

occurs is associated with a granite which intrudes Lower Carboniferous

strata. It is probably to be referred to the close of the Carboniferous

period, and is certainly pre-Triassic. Five different specimens gave

lead ratios in striking agreement, corresponding to an age of 340

million years.

It should be observed in all the following tables that in calculating

the lead ratios the time-average of uranium has been used, and not the

amount actually present. The statement of analyses is, of course, in

percentage.

----------+-------+--------

URANIUM. | LEAD. | RATIO.

----------+-------+--------

70 | 2·9 | 0·041

70 | 3·0 | 0·042

70 | 2·8 | 0·039

72 | 3·0 | 0·041

72 | 2·9 | 0·040

----------+-------+--------

Similar crystals of uraninite have been furnished by the pegmatites of

Branchville, Connecticut. The intruded strata are of either Silurian or

Ordivician age, and the evidence suggests that the period of intrusion

is possibly coincident with that of the earth movements which commenced

at the close of the Ordivician. Here again the lead ratios closely

agree, the age being 430 million years:

----------+-------+--------

URANIUM. | LEAD. | RATIO.

----------+-------+--------

74 | 4·0 | 0·052

75 | 4·0 | 0·051

74 | 4·0 | 0·052

66 | 3·5 | 0·051

----------+-------+--------

In North Carolina uraninite occurs in coarse pegmatites which are mined

for their large flakes of mica. In this instance a good agreement is

scarcely to be expected, as secondary products abound, and the three

specimens from Spruce Pine which were examined by Hillebrand showed

signs of incipient alteration. The fourth specimen was from South

Carolina and this again lacked the freshness which is so essential.

Zircon is also found in the North Carolina pegmatites, and the writer

has examined two sets of specimens with the results given below. The

material in this case appeared to be quite fresh.

The geological period is difficult to establish with certainty. The

relations of the Appalachian rocks of Carolina are so obscure that

the required age may be anywhere from pre-Cambrian to Carboniferous.

Judging from the lead ratios, one is tempted to suggest that the age

is not far from the Silurian, but until our time-scale is better

determined one must be chary in this converse application of the

method. It may confidently be hoped that in this way the geologist will

be greatly helped in his attempt to unravel the history of igneous

activity in the earth’s crust. But the time is not yet, for the

accumulation of facts and data has only just commenced.

-----------+----------+--------+--------------

MINERAL. | URANIUM. | LEAD. | RATIO Pb/U.

-----------+----------+--------+--------------

Uraninite | 77 | 3·9 | 0·049

Uraninite | 77 | 4·2 | 0·052

Uraninite | 67 | 3·3 | 0·047

Uraninite | 71 | 3·3 | 0·045

Zircon | 0·076 | 0·0036 | 0·046

Zircon | 0·130 | 0·0055 | 0·041

-----------+----------+--------+--------------

Other series of minerals could be given from the pre-Cambrian rocks

of North America, Ceylon, and Mozambique, but it is unnecessary to

multiply details further. In all cases the Archean seems to date from

1200-1600 million years.

We now turn to the pre-Cambrian rocks of Scandinavia and Finland, and

for comparison with those of N. America the following classifications

and correlations may be given. The chief unconformities are indicated

by wavy lines. It should be pointed out that the correlation of the

pre-Cambrian rocks over wide areas is, perhaps, the most difficult task

the geologist has to attempt, and the scheme given opposite is based