Three years ago, in describing some basic igneous rocks from Scotland and Ireland, I expressed my conviction that a part, at least, of the structures known as micropegmatitic and granophyric were really of secondary origin. At the same time, I stated that these structures were so much more clearly illustrated by the intermediate and acid rocks of the same district that it would be well to defer the discussion of the subject till my examination and description of the rocks in question should be completed. The study of the intermediate and acid lavas (which, in 1874, I grouped under the old English field-name of “felstones”) and of their plutonic representatives has occupied much of my time and thought during the last fifteen years. But so numerous and interesting are the problems connected with the wonderful metamorphoses which the minerals of these rocks have undergone, and so singular are the changes which have taken place in the structures of these rocks, that, in spite of a number of visits to the district and much labour in the laboratory, some time must yet elapse before the examination of the chief types and the elucidation of their mode of origin is completed. In the meanwhile, however, certain discussions which have taken place concerning rock-structures and their significance—especially as a basis of rock-nomenclature and classification—render it desirable that the very definite and unmistakable evidence which I have obtained concerning the secondary origin of the characteristic structures of the “granophyric” rocks should be placed on