258 PROCEEDINGS OF THE AMERICAN ACADEMY. 



age are small or fail. The hornblende and aegirite also show much 

 the same characters as in the Rattlesnake type although they 

 have been disturbed more by movements in the rock. Fluorite is 

 particularly abundant in the larger aegirites in the form of small 

 included grains. The groundmass is less regular in texture and 

 grain than in the Rattlesnake Hill type and is on the average a 

 little finer. In fact the groundmass ^•aries much from slide to slide 

 and sometimes in the same slide. In some there is an inequigranu- 

 lar mixture of quartz and feldspar with abundant small prismoids 

 of aegirite, with flakes, shreds, fibers and grains of riebeckite; in 

 others the riebeckite predominates to the exclusion of the aegirite. 

 There is often a very fine poikilitic intergrowth of the feldspar and 

 quartz generally attached to the feldspar phenocrysts (see Figure VI, 

 Plate 1). Flow structures are strongly developed in many parts of the 

 groundmass and are less conspicuous or almost wanting in others. In 

 all of the specimens examined, magnetite grains and octahedra were 

 abundant, and many of the larger hornblendes showed signs of con- 

 siderable alteration so that it is certain that part, at least, of the fibers 

 and shreds of riebeckite seen in the groundmass are secondary in 

 origin. It may be noted that the irregularities in the structure of 

 this phase of the porphyry are quite distinctly visible to the eye on 

 well glaciated exposures in the field. They appear as rather faintly 

 marked streaks and patches of slightly differing color and texture, 

 and it is easily discerned that the general direction of the flowage in 

 the rock was parallel to the contact. The aporhyolite on the other 

 hand shows, so far as the writer has observed, no megascopic fiow 

 structure near the contact. 



The band of coarsely porphyritic rock is apparently somewhat 

 variable in width but does not in any case exceed a few feet. It 

 passes rapidly into a rock with a distinctly granular groundmass. 

 The quartz phenocrysts show well round^ed outlines and the "rhom- 

 ben"-like hal)it of the feldspars persists. Their size is on the average 

 about the same as in the preceding phase and is very close to that of 

 the larger feldspar grains of the granite itself. The hornl)lende 

 becomes distinctly niore granitic in hal)it, though it still includes the 

 groundmass grains; the groundmass generally begins to assume a 

 finely granular texture. 



As one recedes still further from the contact the demarkation be- 

 tween phenocrysts and groundmass becomes less and less distinguisha- 

 ble and the minerals assiune gradually the relations to each other 

 found in the porphyritic type of the granite and this, as noted earlier, 



