Principles of Geology

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1835. Excerpt: ... been examined from those tropical seas where they occur in the greatest profusion. Fossil remains of testacea of chief importance.--The testacea then are by far the most important class of organic being3 which have left their spoils in the subaqueous deposits; and they have been truly said to be the medals which nature has chiefly selected to record the history of the former changes of the globe. There is scarcely any great series of strata that does not contain some marine or freshwater shells, and these fossils are often found so entire, especially in the tertiary formations, that when disengaged from the matrix, they have all the appearance of having been just procured from the sea. Their colour, indeed, is usually wanting, but the parts whereon specific characters are founded remain unimpaired; and though the animals themselves are gone, their form and habits can generally be inferred from the shell which covered them. The utility of the testacea, in geological classification, is greatly enhanced by the circumstance, that some forms are proper to the sea, others to the land, and others to fresh water. Rivers scarcely ever fail to carry down into their deltas some land shells, together with species which are at once fluviatile and lacustrine. The Rhone, for example, receives annually, from the Durance, many shells which are drifted in an entire state from the higher Alps of Dauphiny, and these species, such as Bulimus montanus, are carried down into the delta of the Rhone to a climate very different from that of their native habitation. The young hermit crabs may often be seen on the shores of the Mediterranean, near the mouth of the Rhone, inhabiting these univalves, brought down to them from so great a distance. At the same time that some freshwater ...