Page:The Journal of geology (IA journalofgeology21894univers).pdf/18

naturally suggested a comparison of these rocks with those of similar character in the Boston basin and eastern Canada, as well as a further search for other regions of the same kind. This search has already proved successful in North Carolina and Maine, while an examination of the older literature indicates many other places where a recurrence of like conditions may be confidently expected.

The proper interpretation and areal mapping of all the demonstrably volcanic regions in the Appalachian crystallines will not only afford much material of interest in the study of petrography and dynamometamorphism, but will also contribute to the differentiation and final understanding of the vast belt of diverse crystalline rocks to which they belong.

There is notable in the different countries where geology is cultivated a wide diversity of opinion regarding ancient volcanic rocks. In some regions such rocks have been entirely overlooked or else misinterpreted; in others they are recognized, but are conceived as having been formed under circumstances so different from those which now obtain that they are genetically and inherently distinct from the products of modern volcanoes; in a few only are they considered as having been originally identical with recent effusive rocks, and as differing from them only in alterations due to subsequent causes. This diversity of opinion may be accounted for in part by the varying state of preservation of ancient volcanic material in different parts of the earth's surface or by the lack of experience of field geologists with the characteristic features of modern lavas. It is, however, also due in a measure to the persistence of certain ideas promulgated by early masters of the science in their respective lands.

It was in Great Britain that the real nature of ancient volcanic products received its earliest and fullest recognition. In spite of the absence of active volcanoes from the islands, these rocks have from the earliest days of geological inquiry been favorite