Travelers are inveterate collectors of mementos, and those who travel up
and down and across the Connecticut Valley and who delve into its
geologic history may well be interested in gathering records of its
past. The best records are not in notes or printed pamphlets--not even
in this volume on the subject; they are to be found imprinted in the
rocks and minerals themselves. But the value of records is measured
solely by their utility, and utility is achieved by systematic
arrangement. So the authors will venture a few suggestions on collecting
and arranging the minerals and rocks which are present in the valley and
in the bordering uplands.
One mineral may come from a vein, which is the record of a fissure
beneath a hot spring; another comes from a dike, which was a molten
igneous rock. This specimen is a conglomerate or consolidated gravel
washed into place by an ancient stream; that is a slate which was
transformed from clay by intense squeezing and shearing. And if these
four specimens were to constitute the nucleus of a collection, the need
for classification is apparent. The first two are minerals, which are
substances of limited chemical composition and well defined physical
properties. The last two are rocks, which are aggregates of minerals or
of mineral grains. And the minerals may be further classified according
to their separate modes of origin. So, too, with the rocks. Their
mineral composition indicates some of the conditions which existed where
the minerals originated; the shapes of the mineral grains reveal the
process which moved them to their present site; and the arrangement of
grains discloses the conditions existing during aggregation at this new
locality. Mineral make-up, size, shape, and arrangement of the grains
provide means of recognizing major rock varieties--namely, sedimentary,
igneous and metamorphic types,--and also of reading each rock's history.