THE CRYSTALLINE ROCKS NEAR ST. JOHN, N. B. 29 



This is a basic rock, very coarse grained, black and 

 heavy. It is composed of plagioclase feklspar, hypers- 

 thene, olivine and augite in variable quantities, and 

 is therefore to be called an olivine-gabbro. The last three 

 minerals, all unstable ones, have been changed more or 

 less to hornblende. By taking difterent specimens, all 

 stages of this change can been seen, some parts being 

 almost fresh, others completely changed to a hornblende- 

 plagioclase rock, which for the sake of distinguishing it 

 from an igneous rock, originally composed of these 

 minerals, is called an epi-diorite, or gabbro-diorite. The 

 olivine and hypersthene were not of the right chemical 

 composition to change to hornblende, had they been 

 alone. But wherever they came in contact with the 

 feldspar, the necessary alumina and lime Avas supplied 

 and a hornblende crust or rim formed between the two, 

 and enlarged itself at the expense of both minerals. In 

 thin sections this gives a very pretty effect ; the pale 

 brown olivine is surrounded by a rim of bright green 

 needle-like crystals of hornblende, radiating out into the 

 colorless feldspar. 



The little area at Indiantown, close to the end of 

 Main street, and south along the water-front, fur7iishes a 

 very good example of segregation. At its south end it 

 is composed chiefly of feldspar, with very little of the 

 dark colored silicates, ':,nd is a greyish white rock, with 

 dark spots. As we go toward the north end, the dark 

 silicates increase in amount and the feldspar decreases, 

 till finally the rock is made up almost entirely of olivine, 

 hypersthene and augite, and is of a dense and glistening 

 brownish black color. 



The Volcanic Rocks. — Bstwoen the Laurentian lime, 

 stones and the Cambrian slates lies a rather narrow bund 



