THE MONTEREGIAN HTLLS 



275 



Diagrams showing the composition of these several rocks are 

 presented in Fig. 4. 



The transitional rock. — As has been mentioned, there inter- 

 venes in Mount Johnson between the pulaskite border and the 

 central mass of essexite a transitional zone consisting of a rock 

 which is dark in color and 



At rt 



thus resembles the essexite, 

 but which is characterized 

 by the presence of large por- 

 phyritic feldspars sometimes 

 as much as two inches in 

 length, of peculiar form scat- 

 tered through it and often 

 arranged with their larger 

 axes in the same direction, 

 thus giving a fluidal appear- 

 ance to the rock. This rock 

 contains a large proportion 

 of the same iron-magnesia 

 minerals, more especially 

 the hornblende, found in the 

 essexite, and passes over 

 gradually into this rock. Its 

 passage into the pulaskite is 



llather more abrupt and is Fig. 4.— Diagrammatic representation of 



marked chiefly by the almost the chemical composition of the several rocks 



described. 



No. I. Laurvikose — Mount Johnson. 



No. 2. Laurvikose — Shefford mountain. 

 No. 3. Nordmarkose — Shefford mountain. 

 No. 4. Andose — Mount Johnson. 

 No. 5. Essexose — Mount Johnson. 

 No. 6. Andose — Shefford mountain. 



entire disappearance of the 

 dark-colored constituents 

 above mentioned. There is, 

 however, a continuous tran- 

 sition or passage from the 

 pulaskite through this inter- 

 mediate rock into the inner essexite of the mountain. 



This transitional rock is composed of the same minerals as 

 the essexite with the exception of the feldspar, which consists in 

 part of the soda-orthoclase characteristic of the pulaskite, and in 

 part of the plagioclase (in this case oligoclase) which forms the 



