the outer slope of the crater rim is found to be covered with blocks
of lava. A black, loose-textured andesite is most abundant, and from
its occurrence on the edge of this well-defined crater may be regarded
as representing the later eruptions of Rainier. Lower down on the
slopes of the mountain opportunities for the study of the structure of
the volcanic cone are found in the bold rock masses that mark the
apexes of the interglacial areas. Examples of these are Little Tahoma,
Gibraltar, Cathedral Rock, the Wedge, and the Guardian Rocks. These
remnants of the old surface of the cone, together with the cliffs that
bound the lower courses of the glaciers, exhibit the structural
relations very well.
Even when viewed from a distance these cliffs and peaks are seen to be
composed of bedded material. Projecting ledges interrupt the talus
slopes and express differences of hardness in the several beds, while
variations in color also indicate separate lava flows and agglomeratic
deposits. Gibraltar is thus seen to be composed of interbedded lavas
and pyroclastics, and on the Wedge a similar alternation is several
times repeated, a pink agglomerate being exceptionally striking in
appearance.
These lava flows and beds of volcanic ejectamenta thus exposed dip
away from the summit at a low angle. The steepest dip observed was in
the amphitheater at the head of Carbon Glacier, where in the dividing
spur the dip to the northeast is about 30°. Some exceptions in the
inclination of the beds were noted on the southeastern slope, where in
a few cases the layers are horizontal, or even dip toward the central
axis of the cone. In general, however, the volcanics composing Mount
Rainier may be said to dip away from the summit at an angle somewhat
lower than that of the slopes of the present cone. In the outlying
ridges to the north, the Mother Range, Crescent Mountain, and the
Sluiskin Mountains, the structure seems to be that of interbedded
volcanics approximately horizontal. The extent of the volcanics from
the center of eruption has not been determined. Similar lava extends
to the south, beyond the Tattoosh Range, and volcanics of similar
composition occur to the north, in the Tacoma quadrangle. The latter
lavas and tuffs may have originated from smaller and less important
cones, now destroyed by erosion.
A radial dike was observed at only one locality, near the base of
Little Tahoma. In several cases the lava masses, as seen in cross
section, are lens-shaped, and where associated with fragmental beds
have unconformable relations. This shows that some of the lava flows
took the form of streams, relatively narrow, rather than of broad
sheets. Such a feature is in accord with the distribution of rock
types. Thus along Ptarmigan Ridge for considerable vertical and
horizontal range the rock shows only slight variation. The
distribution of rock types will be more fully discussed in a later
paragraph.
Of how large a part of the lava flows the crater still remaining was
the point of origin is a question to be answered only after more
detailed observation has been made. The best section for the study of
the succession of flows and ejectamenta is the amphitheater at the
head of the Carbon Glacier. The 4,000 feet of rock in this bold wall
would afford an excellent opportunity for this were it not that
frequent avalanches preclude the possibility of geologic study except
at long range.
MEGASCOPIC CHARACTERS
The volcanic rocks of Rainier are of varying color and texture. Dense
black rocks with abundant phenocrysts of glassy feldspars, rough and
coarse lavas of different tints of pink, red, and purple, and compact
light-gray rocks are some of the types represented upon the slopes of
this volcanic cone. In color, the majority of the rocks may be grouped
together as light gray to dark gray. The black and red lavas are less
common. In texture, the Rainier lavas are, for the most part, compact.
Slaggy and scoriaceous phases are common, but probably represent only
a small part of the different flows. Near the Guardian Rocks large
masses of ropy lava are found which suggest ejected bombs.
Agglomeratic and tuffaceous rocks are of quite common occurrence,
although less important than the lavas. Vesicular lavas occur at
several localities, and fragments of a light-olive pumice, many as
large as a foot in diameter, wholly cover some of the long, gentle
slopes southeast of Little Tahoma and in Moraine Park.
Contraction parting or jointing is often observed, being especially
characteristic of the basaltic types. The platy parting is the more
common, but the columnar or prismatic parting is well exhibited at
several localities. The black basaltic lava east of Cowlitz Glacier
shows the latter structure in a striking manner. The blocks resemble
pigs of iron in size and shape, and where exposed in a vertical cliff
these seem to be piled in various positions.
The rocks on the higher slopes of Mount Rainier are in general very
fresh in appearance. An exception may be noted in the case of the
rocks at the base of Little Tahoma, where some alteration is evident.
The bright coloring of the surfaces of the lava blocks and the general
appearance of the face of the cliff may indicate fumarole action at
this point. There is also some decomposition along the inner edge of
the crater rim, near the steam vents. On the lower slopes, some
distance below the snow line, the freshness of the rock is not a
noticeable feature, and it is seen that here weathering is of the
nature of chemical decomposition as well as of mechanical
disintegration.
MICROSCOPIC CHARACTERS
Microscopically these lavas show more uniformity than is apparent
megascopically. Rocks which in color and texture appear quite diverse
are found to be mineralogical equivalents. The majority of these rocks
are andesites, the hypersthene-andesites predominating, as was shown
by Hague and Iddings; but over large areas the andesites are decidedly
basaltic, and, indeed, many of the lavas are basalts. The megascopic
differences are mostly referable to groundmass characters, the color
of the rock being dependent upon the color and proportion of glassy
base present. Therefore the degree of crystallization of groundmass
constituents is of more importance in determining the megascopic
appearance than is the mineralogical composition, and the basaltic
lavas are for the most part light gray in color, while the more acid
hypersthene-andesites are often black or red.
In petrographic character the lavas range from hypersthene-andesite to
basalt. This variation is dependent upon the ferromagnesian
silicates, and four rock types are represented--hypersthene-andesite,
pyroxene-andesite, augite-andesite, and basalt--any of which may carry
small amounts of hornblende. A rigid separation of these rock types,
however, is impossible, since insensible gradations connect the most
acid with the most basic. In the same flow hypersthene-andesite may
occur in one portion, while in close proximity the lava is an
augite-andesite.
These lavas have groundmass textures that vary from almost
holo-crystalline to glassy. The felted or hyalopilitic texture is the
most common, and plagioclase is the principal groundmass constituent.
The feldspars are lath-shaped, often with castellated terminations. In
the more basic phases anhedrons of augite and of olivine appear, and
magnetite grains are usually present. Flowage is often beautifully
expressed by the arrangement of the slender laths of feldspar.
Among the phenocrysts feldspar is the most prominent. It has the usual
twinning characteristic of plagioclase and belongs to the
andesine-labradorite series, extinction angles proving basic andesine
and acid labradorite to be the most common. Zonal structure is
characteristic, being noticeable even without the use of polarized
light. Zonal arrangement of glass inclusions testifies to the
vicissitudes of crystallization, and often the core of a feldspar
phenocryst is seen to have suffered corrosion by the magma and
subsequently to have been repaired with a zone of feldspar more acid
in composition.
Of the darker phenocrysts, the pyroxenes are more abundant than the
olivine or hornblende. Hypersthene and augite occur alone or together,
and are readily distinguished by their different crystallographic
habits as well as by their optical properties. The hypersthene is
usually more perfectly idiomorphic and occurs in long prisms, with the
pinacoidal planes best developed, while the augite is in stout
prisms, usually twinned. Both are light colored, and the pleochroism
of the hypersthene is sometimes quite faint. According to the relative
importance of these two pyroxenes, the lavas belong to different
types, hypersthene-andesite, pyroxene-andesite, or augite-andesite.
Olivine occurs in certain of the Rainier lavas, in stout prisms
somewhat rounded and often with reddened borders. The usual
association with apatite and magnetite crystals is noted. The olivine
varies much in relative abundance, so as to be considered now an
accessory and now an essential constituent, and in the latter case the
rock is a basalt.
Hornblende is not abundant in any of the rocks studied, although
typical hornblende-andesite has been described among the specimens
collected by Professor Zittel. Where it occurs it is in brown
crystals, which have usually suffered magmatic alteration. In one
case, where this alteration is less marked, the idiomorphic hornblende
is found to inclose a crystal of labradorite, and thus must have been
one of the latest phenocrysts to crystallize. It also surrounds
olivine in this same rock,[29] which is a hypersthene-andesite, the
hornblende and olivine being only accessory.
The different textures of these lavas are doubtless expressive
primarily of diversity in the physical conditions of consolidation,
but also in part of variations in chemical composition. The variations
in mineralogical composition are likewise referable to these two
factors, but here the latter is the more important. The
hypersthene-augite olivine variation, already referred to, doubtless
well expresses the chemical composition of the magma, and deserves to
be taken as the chief criterion in the classification of the lavas. As
was noted by Hague and Iddings, the hypersthene and olivine play a
like rôle, the former occurring when the silica percentage is somewhat
higher than in basalt. It is exceptional to find the two in the same
specimen, the one being absent whenever the other is present. The
following analysis[30] of the typical hypersthene-andesite from Crater
Peak shows the lava to be a comparatively acid andesite:
ANALYSIS OF HYPERSTHENE-ANDESITE FROM CRATER PEAK, MOUNT RAINIER
PER CENT.
SiO_{2} 61.62
Al_{2}O_{3} 16.86
FeO 6.61
CaO 6.57
MgO 2.17
Na_{2}O 3.93
K_{2}O 1.66
-----
99.42
An analysis[31] of one of the light-gray, olivine-bearing rocks on the
northern slope of the mountain gives a silica percentage of 54.86, and
is doubtless representative of the more basic of the Rainier lavas.
The sporadic occurrence of hornblende in these andesites is
principally the result of physical conditions rather than of chemical
composition. The magmatic alteration of the phenocrysts of hornblende
affords evidence of this variation in consolidation conditions, a
diminution of pressure with continuance of slow cooling giving rise to
the magmatic alteration of the hornblende. That this change took place
during the later stages of consolidation is shown by the relative age
of the hornblende, noted above, and also by the fact that in one case
a phenocryst of augite, where it abuts against the hornblende, has
protected the latter from this alteration. The alteration is in part
pseudomorphic, the hornblende retaining its characteristic outlines,
but often there has been resorption. In one andesite the abundance of
these remnants of hornblende and also of augite anhedrons in the
groundmass may justify the conclusion that this augite andesite is of
derivative origin, of the class described by Washington.[32] It may be
noted also that hypersthene shows a tendency to magmatic alteration,
although only rarely.
In a basal flow in Moraine Park, the slaggy and compact phases show
differences in phenocrysts as well as in groundmass. The glassy rock
has hypersthene as the predominant phenocryst, while feldspar is the
more important in the compact and more crystalline andesite.
The distribution of the rock types described above is of interest. On
the northern slope of the mountain, between Willis and Carbon
glaciers, the characteristic lava is a gray andesite, smooth to rough
in texture, and showing platy and columnar parting. Hypersthene is not
the prevailing pyroxene, and olivine is usually present, often in such
abundance as to make the rock a basalt.
In Moraine Park gray andesites also predominate, with both pyroxenes
as phenocrysts, but here hypersthene is the more important. On the
eastern slope on the Wedge, between Winthrop and Emmons glaciers, the
lavas are pyroxene-andesites and vary much in megascopic appearance,
although little in microscopic characters. These rocks are quite
distinct from any seen to the north. The nunatak in Emmons Glacier is
composed of hypersthene-andesite, but on Little Tahoma the lava shows
more variety. Both augite-andesite and hypersthene-andesite occur,
while at the southern end of this interglacial rock mass, just east of
Cowlitz Glacier, the cliffs are composed of the prismatic black
basalt. On Crater Peak, and below on Gibraltar, hypersthene andesite
occurs with considerable variation of color and texture. On the spurs
west of Nisqually Glacier the andesites contain both pyroxenes, the
augite being somewhat the more important.
The distribution of the volcanic rocks, as determined in the study of
reconnaissance collections, indicates that the cone has been built up
by eruptions of lava and of fragmental material. The successive lava
streams were doubtless of considerable thickness, but were limited in
lateral extent. The beds of fragmental material are of the nature of
flow breccias and of coarse agglomerates on the higher slopes, while
tuffs occur at a greater distance from the center of eruption. This
composite cone appears to be remarkably free from radial dikes, which
may indicate that the volcanic energy was expended chiefly at the
crater. The variation in rock types on different sides of the volcanic
cone may be evidence of changes in position of the center of eruption.
The destruction of an earlier crater and the eccentric position of a
later would give rise to such a radial distribution of lavas as has
been described above.
GRANITE
OCCURRENCE
The presence of an acid holocrystalline rock on the slopes of Mount
Rainier was first reported by Lieutenant Kautz in 1857, from whose
accounts Dr. George Gibbs was led to announce the occurrence of
granite as a dike in recent lavas.[33] Emmons in 1870 observed a cliff
of "beautiful white syenitic granite" rising above the foot of
Nisqually Glacier and correctly interpreted the geologic relations. In
1895, on a reconnaissance trip, the writer identified granite among
the bowlders composing the lateral moraines of Carbon Glacier, as
well as on the surface of the glacier itself, and in the following
season bowlders of granite were found to be plentiful in the river bed
at the foot of this glacier. This anomaly of granite bowlders coming
from a volcanic peak was also noted in the canyon of the Nisqually by
Emmons.
In the somewhat more careful study of the Mount Rainier rocks, search
was made and the granite was found in place at several points on the
northeastern slope. A biotite-hornblende-granite was observed on
Carbon River at the mouth of Canada Creek, about 12 miles from the
summit of Mount Rainier, and at Chenuis Falls, 2 miles up the river, a
finer grained holocrystalline rock occurs, apparently an aplitic phase
of the granite. In the lower portion of Carbon Glacier, near its
eastern edge, a nunatak of granite can be seen, while the same rock
occurs farther to the east, beyond the older of the lateral moraines.
Higher on the slopes of Rainier a more marked ridge of granite was
traced. A knob rises above the eastern moraine of Carbon Glacier at an
altitude of between 7,000 and 8,000 feet, and the more prominent
features to the east in Moraine Park also owe their survival to the
greater erosion-resisting power of the granite.
PETROGRAPHIC DESCRIPTION
These granites have few features worthy of special mention. Hornblende
and biotite are the ferromagnesian constituents and vary much in
relative importance. The variations from hornblende-granite to
biotite-granite occur in the same knob or ridge, and considering all
occurrences the two varieties seem to be of equal development. There
is also some variation in the amount of quartz present, and in the
relative importance of the orthoclase and plagioclase. All of these
characters are also found in the granites of the Northern Cascades.
RELATION TO THE VOLCANIC ROCKS
Along the side of the knob overlooking Carbon Glacier the granite as
seen from a distance appears to be intrusive. Blocks of andesite cover
the slope, deposited there by the glacier at a time when it possessed
greater lateral extent, and the granite talus from above crosses this
same slope in a narrow band. The relations prove less deceptive on
close examination, and the granite is seen to constitute an older
ridge. Farther along this ridge, at the cliffs on the north-eastern
edge of Moraine Park, the granitic rock is found over-lain by the
lava. The actual contact of the two rocks is concealed by soil filling
the crevice left by disintegration along the contact plane. The
granite, however, exhibits no intrusive characters, while the
overlying andesite becomes scoriaceous in its lower portion, although
compact immediately above. This contact is on the southern side of the
granite ridge, the crest of which is approximately east-west. This
position of the lava contact considerably below the highest occurrence
of the granite indicates that the topographic features of this old
granite ridge were even more marked at the time of the eruption of the
lavas and the building of the volcanic cone. Above this ridge of
granite on the one side tower the cliffs of bedded volcanics which
compose the Sluiskin Mountains, and on the other is the andesite ridge
bounding the canyon of Winthrop Glacier. Thus Mount Rainier, although
a volcanic peak, rests upon an elevated platform of granite which is
exposed by erosion at a few points on the slopes of the mountain.
SUMMARY
The volcanic rocks of Mount Rainier include both lavas and
pyroclastics. The breccias, agglomerates, and tuffs, although of
striking appearance, are, perhaps, less important elements in the
construction of the composite cone.
The lavas vary much in color and texture, but these megascopic
differences are referable rather to the degree of crystallization of
the magma than to its chemical character. The variation in the chemical
composition of the lavas expresses itself in mineralogical differences,
and thus four rock types are distinguished--hypersthene-andesite,
pyroxene-andesite, augite-andesite, and basalt. The distribution of
these types indicates a radial arrangement of lava streams, and
hypersthene-andesite is the more abundant variety of lava.
Granite is exposed on the slopes of Rainier where erosion has cut away
the overlying lava, and it is plain that the volcanic cone rests upon
an elevated platform of older rock, approximately 8,000 feet above sea
level.
[Illustration: _Copyright by Harris & Ewing, Washington, D. C._
PROFESSOR CHARLES VANCOUVER PIPER]
XVI. THE FLORA OF MOUNT RAINIER
BY PROFESSOR CHARLES V. PIPER
Charles Vancouver Piper was born on Vancouver Island, at
Victoria, British Columbia, on June 16, 1867. He graduated
from the University of Washington in 1885 and since then has
received degrees and honors from other institutions and
learned societies. He was professor of botany and zoölogy at
the Washington Agricultural College (now State College of
Washington) from 1892 to 1903. He has been agrostologist in
charge of forage crop investigations for the Bureau of Plant
Industry, United States Department of Agriculture, since
1903.
He has discovered many new forms of plant life and has
published many monographs and books in the field of botany.
This account of the flora of Mount Rainier was first
published in The Mazama (Portland, Oregon) in two articles,
one in Volume II, Number 2 (April, 1901), and the other in
Volume II, Number 4 (December, 1905). They are reproduced
with the consent of the editor of The Mazama, and Professor
Piper has revised and amplified them for this purpose.
Up to an elevation of 4,000 feet or more the flanks of Mount Rainier
are clothed in a continuous belt of somber forest, broken only where
glaciers and their nascent streams have hewn pathways, or where, alas,
fire has left desolate slopes marked here and there by the whitened,
weather-worn shaft of some old tree, a dreary monument to its
destroyed fellows. This forest is composed in its lower reaches
largely of Douglas spruce. Scattered through it in smaller quantities
one finds Lovely fir, Western white pine, Western hemlock, a few
Engelmann spruces, and on the stream banks cedar and yew, and now and
then a little cottonwood.
At about the 3,500-foot level the character of the forest changes. The
Western hemlock gives way to the larger-coned Black hemlock; the
Douglas spruce and Lovely fir are replaced by the Noble fir; and the
ragged-barked Alaska cedar greets the eye. Another thousand feet and
the Subalpine fir replaces its two near relatives. From this point
upward, the forest, now composed only of Black hemlock, Alaska cedar
and Subalpine fir, to which in some places the White-bark pine must be
added, is confined largely to the crests of ridges and straggles up
the mountain in irregular broken lines. Between these timbered ridges
extensive grassy slopes appear, veritable flower gardens when in their
glory.
At 6,500 feet elevation the timber ceases to be. Scraggly prostrate
firs and hemlocks, sprawling as it were on the earth for shelter, mark
sharply the limit of their endurance. Here, too, the continuous carpet
of grass and flowers ceases--and a soil of volcanic sand or powdered
pumice supports a very different vegetation. At 10,000 feet the
toughest mountaineer of all the flowering plants, _Smelowskia ovalis_,
still appears. Far above this, however, even to the crater's rim,
lichens trace their hieroglyphics on the rocks; and on the
steam-warmed rocks of the crater two mosses find lodgment, _Hypnum
elegans_ Hooker?, and _Philonotis fontana_ Bridel, the latter even in
fruit.
Few plants grow in the dense shades of the lower forests, and these
are mainly ericaceous. Most plentiful are _Vaccinium ovalifolium_, _V.
macrophyllum_, _Gaultheria ovatifolia_, _Menziesia ferruginea_,
_Pachystima myrsinites_, _Cornus canadensis_ and _Clintonia uniflora_.
Here, too, occur several weird-looking whitish or reddish saprophytes,
_Monotropa hypopitys_, _Pterospora andromedea_, and _Corallorhiza
mertensiana_.
On the drier portions of the grassy slopes _Lupinus subalpinus_,
_Castilleja oreopola_, _Potentilla flabellifolia_, _Pulsatilla
occidentalis_, _Erigeron salsuginosus_, _Polygonum bistortoides_,
_Phyllodoce empetriformis_, _Cassiope mertensiana_ and _Vaccinium
deliciosum_ are the most attractive plants. Where the ground is
springy _Veratrum viride_ occurs in great clumps and _Dodecatheon
jeffreyi_, _Caltha leptosepala_ and _Ranunculus suksdorfii_ are
plentiful.