National Geographic : 1892 Mar 21
Definition of Rock Types. (rhyolite?), hornblende-porphyrite, augite-porphyrite and dia base.* COARSE-GRAINED PLUTONIC RocKS. Not only does the structure of the more coarsely granular rocks examined by me from Alaska indicate that they originated, as above explained, under very different physical conditions from the more finely grained porphyritic specimens, but another of their characteristics renders it probable that they are also much more ancient than these. This is the evidence of extensive dynamic action to which they have been subjected, manifested in the fracturing, optical derangement, granulation, or meta morphism of the constituent minerals. The absence of such phenomena from the dike rocks is the second and more important reason for correllating the coarser specimens with a geologically earlier and possibly Archean terrane. These plutonic rocks will be considered under the heads of diorite and gabbro. Diorite. Nine of the specimens collected by Professor Reid from the foot of the Muir glacier are representative diorites. They are numbered 1, 2, 3, 4, 5, 6, 7, 15 and 22. All are well preserved and differ from one another principally in the coarseness of their grain, in the evidences which they exhibit of dynamic action, and in their proportions of accessory pyroxene or biotite. Augite-diorite.-Numbers1 and 2, evidently identical, are rather coarse-grained augite-bearing diorites. Their principal constitu ent visible to the unaided eye is dark green hornblende, whose cleavage surfaces reach two centimeters in diameter. The spaces * The terms porphyry and porphyrite must be understood as used throughout this paper in a purely structural sense, with no reference to either geologic age or secondary alteration. In classifying a collection like this one, any reference to geologic age is plainly out of the question, and the specimens, while often much altered, still show the original structure of their groundmass with distinctness. This is in almost every case holo crystalline, and often quite coarsely crystalline, indicating in general a slower rate of cooling than that common to purely surface rocks. In this sense only then are the porphyries and porphyrites herein described sup posed to differ from their less crystalline and more superficial equivalents, the rhyolites and andesites. In this usage I profit by the counsel of my friend, Mr J. P. Iddings, whose extensive researches among kindred rocks entitle him to speak with authority upon such a point.
1892 May 15
1892 Feb 19