Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/410

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Dr. Mac Culloch on the Geology of

cution, and reference to those well-known members which they may chance to resemble. If in these cases we apply the name of any of their species in place of a generic term, we render our description of individuals unintelligible, and often contradictory and absurd. Thus the name basalt has been given to greenstone, as well as to many other modifications of trap, to which no name has, or perhaps ever can be applied. But the chief confusion has resulted from the application of the term greenstone, as a generic term for these rocks. In this way we have had greenstones of all colours, from red to white, and in which neither hornblende nor felspar were to be distinguished.

It would assuredly be better in describing the geological appearances of these rocks, when the description of individuals is not necessary, to make use of a generic term which should distinguish the character of the family, without confounding the individuals. The peculiar aspect of the rocks of this family renders the name of trap as expressive as any which can be applied, and in this sense, if it be limited to that well-known class of unstratified rocks which includes the above-named individuals and their numerous modifications, we shall escape the confusion which must otherwise ensue, and which unfortunately already exists too much.

It is evident that the other modifications of this rock consisting of amygdaloid and of the various trap porphyries, are still further subordinate species, if I may style them so, ranking according to their particular basis, under the several species above enumerated.

It is said that in the Shiant islands ammonites and other animal remains are discovered in beds under the trap, but I did not observe them. They may very probably however exist here, as they are of frequent occurrence in Sky.

Among the rolled pebbles on the shore I met with specimens of