﻿450 Canadian Record of Science. 



to intense pressure. In this class, however, as in many other classes 

 of rocks, the work of the investigator is by no means ended, and the 

 future will no doubt settle the problems concerning their origin, which 

 are to-day unsolved. 



The anorthosites belong to the gabbro family of igneous plutonic 

 rocks, and occur in the Archaean as intrusive masses of all sizes, which 

 masses break through the gneisses in various localities, from Labrador 

 to Lake Superior. Sir William Logan and others, who had examined 

 the Morin intrusion in this area, held that the anorthosites were of 

 sedimentary origin, because they possessed a foliated structure, which 

 was considered at that time to be the remains of a partly obliterated 

 l)edding. The formation was then called the Upper Laurentian, or 

 Norian. 



The result of modern research, however, shows that this foliated 

 structure is of a secondary character and due to movements in the 

 rock, brought about by the great pressure to which they have been 

 subjected when deeply buried under a great mass of overlying rock. 



As the Morin mass is by far the most important to this area it can 

 be taken as a typical example. Anorthosite is composed essentially of 

 nearly pure plagioclase felspar with accessory minerals in small 

 amount, the most important being augite, hypersthene and ilmenite, 

 the latter of which occurs in concentrations of large size. 



According to the structure of the rock, it is divided into granulated 

 and non-granulated, but as in the case of the gneisses there are many 

 intermediate forms. The former variety is developed in the main 

 mass, while the latter occurs along the borders of the mass and 

 especially in the south-east arm which passes under the Palaeozoic. 



In the non-granulated rock the plagioclase is fresh arid generally of 

 a dark violet color, which, when examined microscopically, was found 

 to be due to numerous rod-like inclusions. When these are absent the 

 felspar is white. These rods are supposed to be composed of titanifer- 

 ous iron. The felspar, which is a labradorite, is usually twinned and 

 sometimes has as alteration products calcite, epidote, zoisite and 

 saussurite. The augite is fresh and of a light green color, often seen 

 growing around iron ore ; the hypersthene closely resembles the 

 augite. The iron ore is ilmenite, being exceedingly rich in titanic 

 acid as contrasted with tlie iron of the Laurentian, which has none or 

 very little. The microscope shows that the ilmenite is often found 

 closely associated with magnetite in small amount. The rock 

 examined in situ is very coarse in grain and contains dark patches or 

 streaks, which are caused by a concentration of bisilicates and iron 

 ore. On the same structure in rocks ai'ound Baltimore Dr. E. H. 

 Williams remarks as follows: — " Tlie most striking feature in the 

 texture of the unaltered gabbro is the repeated and abrupt change in 

 the coarseness of the grain which is seen at some localities. It was 



