Mineral and Rock Collections
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.