THE CRYSTALLINE ROCKS NEAR ST. JOHN, N. B. 25 



viuscovite, is a potash-alumina silicate, and the other, 

 bf'olite, is a magnesia-iron alumina silicate. The first is 

 rarely seen in igneous rocks, except a few very light 

 colored granites. The second, which is much darker in 

 color, is a common constituent of them. Hornblende and 

 augite are silicates of alumina, lime, and magnesia and 

 iron. Hyjjersthene and olivine are magnesia-iron silicates. 

 Orthoclase feldspar, and some varieties of plagioclase, 

 contain a very large proportion of silica, while hypersthene 

 and olivine contain less than the rest. All these minerals 

 are of definite composition, but one basic oxide may 

 replace another to a certain extent. Magnetite, an oxide 

 of iron, is found in noticeable amount in the more basic 

 rocks. 



We name the rocks according to the minerals most 

 abundant in them, and also according to their structure, 

 which is granitic in those which have cooled at a con- 

 siderable depth, porphyritic, or glassy, in surface-cooled 

 masses. Among the granitic rocks we have a series 

 from acid to basic, as follows : Granite, composed of 

 quartz, orthoclase, and usually some plagiolase, and a 

 small amount of mica, or hornblende, or both. This 

 grades into Quartz-diorite, composed of quartz, plagio- 

 clase, and often some orthoclase, and with rather more 

 hornblende, or mica. By the quartz decreasing and 

 hornblende increasing, we get a plagioclase-hornblende 

 combination, or diorite. This, again, by augite replacing 

 the hornblende, becomes a gabbro, and the gabbro, with 

 incoming of hypersthene and olivine, becomes an olivine- 

 gabbro. This, finally, by decrease of the feldspar, grades 

 into aperidotite, composed of olivine with hypersthene, or 

 augite, or both. 



Corresponding to these intrusive rocks we have dif- 

 ferent volcanic rocks, which are not so easy to distinguish, 



