INTERESTING LOCALITIES OF THE SIAMO SLATE. 323 



schistose, is not strongly foliated. In general the schistosity dips to the 

 south, l)ut there are in many cases minor crinkles, and for short distances a 

 northern dip. Just north of the main ridge of gTeenstones the schists are 

 cut at a mimber of jilaces by dikes of greenstone, varying in width from 

 minute ones to those 15 feet across. The larger dikes usually cut across 

 the schistosity, but some of the smaller ones were intruded almost exactly 

 parallel to the schistosity. At one place an earlier, coarse greenstone is 

 cut by a later dike. At one exposure the schistosity has a dome structure, 

 apparently occasioned by intrusive greenstone, which just reaches the sur- 

 face of the ground. Adjacent to the dikes the minor crinklings of the 

 schist are often prominent. At various places the greenstone dikes and the 

 schists are so lirmly welded that the rock breaks elsewhere rather than 

 along tlie contact, and there appears to be a gradation zone a fraction of an 

 inch across l^etweeu the two, but in general the contact is rather sharp. In 

 and upon the main ridge of greenstone to the south are considerable areas 

 of the Siamo slate, which have apparently been caught in the eruptive mass. 

 Near the base of the formation there is seen at one place a continuous expo- 

 sure frona the Ajibik quartzite to the typical mica-slate, and in the ])assage 

 from one to the other there are a number of interlaminations of the quartzite 

 and mica-slate, some of the beds being so thin that a hand specimen shows 

 several of each of the two formations. Here the schistosity developed 

 parallel to the bedding, as shown by its relation to the coarse and fine 

 layers. At one or two places near the base of the formation there are also 

 thin interlaminated belts of garuetiferous griinerite-maguetite-schist. 



As examined in thin section, the most altered phases of the rocks 

 are typical, regularlv laminated, chloritic and sericitic biotite-schists, but, 

 as explained in the general description, there are less altered phases in 

 this area which distinctly show the fragmental character of the rock and 

 the manner of transition from one to the other. The mica-schists are found 

 adjacent to the intrusive greenstone, but the mica- slate also has as close 

 relations. The contact between the two rocks, as seen in thin section, is 

 verv sharp, Ijoth in the case of slate or schist cut by dikes and in the case of 

 included blocks in the main mass of greeiistone. Near the contact, within 

 the biotite-schist there are found large crystals of hornblende, which include 



