Definition of Bock Types. 65 



(rhyolite?), hornl:)lende-porphyrite, augite-porphA^rite 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 ver}^ 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 1:)een 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 tlie coarseness of their 

 grain, in the evidences which they exhibit of dynamic action, 

 and in their proportions of accessory pyroxene or biotite. 



Augite-diorite. — Numbers 1 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 structui^e 

 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 sujjerficial 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 Avith authority upon such a point. 



