258 FRANK D. ADAMS 



passes gradually into the other, the whole constituting one intru- 

 sive unit. The approximate limits of these several rock species 

 are shown in the accompanying map (Fig. 2) and photograph 

 (Fig. 3) of the mountain, it being impossible sharply to delimit 

 the several species, seeing that they pass into one another. The 

 mass therefore becomes progressively more basic as we pass 

 from the margin of the intrusion to its center. The two chief 

 rock types are the pulaskite and the essexite which will be sepa- 

 rately considered. The essexite, being the more abundant rock 

 and one presenting a greater complexity in mineralogical com- 

 position, may be first described. 



Essexite. — The rock is dark in color and rather coarse in 

 grain, and although holocrystalline usually presents a more or less 

 marked fluidal arrangement of the constituents. This is espe- 

 cially marked in the zone of transition between the essexite and 

 pulaskite, owing to the presence there of the large feldspar 

 phenocysts which, being arranged with their longer axes parallel 

 to the direction of flow, serve to accentuate this structure. The 

 finer-grained variety forming the summit of the mountain is more 

 massive in character and does not exhibit the fluidal arrange- 

 ment of constituents. Under the microscope the rock is seen to 

 be composed of the following minerals : hornblende, pyroxene, 

 biotite, olivine, plagioclase, nepheline, sodalite, apatite, mag- 

 netite, sphene, and in some cases a very small amount of ortho- 

 clase. 



There is a marked tendency on the part of all the constituents 

 to assume an idiomorphic development. The long lath-shaped 

 plagioclases and large hornblende individuals have an approxi- 

 mately parallel arrangement, and between these lie the other 

 iron-magnesia constituents with the smaller plagioclase indi- 

 viduals, the nepheline and the other components of the rock. 

 These interstitial constituents do not differ greatly in size from 

 the others and show the same tendency to a parallel arrange- 

 ment. 



Hornblende . — Although almost every thin section of the rock 

 contains not only hornblende, but pyroxene and biotite also, 

 their relative proportion varies considerably. The hornblende 



