On the Microscopic Structure and Composition of British Carboniferous Dolerites

Abstract

It will be seen from the preceding descriptions that great variations in texture and composition frequently occur in a single rock, also that they are of the same kind as those which sometimes characterize large rock-masses and to which different names have been applied. An extended inquiry shows, however, that such differences should be regarded as mere local varieties, interesting in themselves, but of quite subordinate importance from a petrological point of view. Any one who has studied these rocks in the field must be aware that specimens may be collected in a single quarry which would be named basalt, anamesite, and dolerite by many petrologists, if taken from different localities; and even in dykes of very moderate dimensions it is by no means unusual to find the central parts coarsely crystalline, while the sides are more compact or cryptocrystalline. Variations in composition are equally common; a collector, for example, who examined a single specimen from Salisbury Crag or Rowley, might conclude that the felspathic constituent is orthoclase, and would be tempted to give a new name to the combination; but an examination of other examples would show that his specimens contained merely exceptional aggregations of orthoclase, and in no way represented the general character of the masses.