612 THE UPPER SILURIAN. ^ 



in their production, and also as to the question whether they are 

 derived from deep-seated sources under the stratified rocks, or have 

 been produced from the fusion or semi-fusion, with aid of heated water, 

 of portions of the sediments themselves. That in some cases they are 

 at least partially of the latter character is, I think, evident from the 

 manner in which they appear to replace stratified rocks in their line 

 of strike, or to occur in bedded masses among them, and also from 

 their apparent relation in mineral character to the associated igneous 

 rocks. On the other hand, in their frequent intrusion into the fissures 

 of the aqueous rocks, they are certainly in many cases to be regarded 

 as eruptive rocks of unknown and deep-seated origin. Dr Hunt has 

 very fully considered these points in reference to similar rocks in the 

 Reports of the Geological Survey of Canada. In the district now 

 under consideration, the following terms may serve for the designation 

 of the more important rocks of this class : — 



Unstratijied Diorite or Hornhlcndic Gi^eenstone consists of horn- 

 blende and felspar, sometimes in large distinct crystals of black or 

 green hornblende and white felspar, and in every gradation of crystal- 

 line texture between this and a gray or greenish rock in which the 

 separate crystals are scarcely distinguishable. When there are large 

 distinct felspar crystals imbedded in the mass, it is named Porphyritic 

 greenstone. 



Unstratijied Compact Felspar or Felsite is a rock consisting of the 

 materials of felspar with some quartz, but not distinctly crystalline. 

 It is sometimes fine-grained and flinty in aspect, and in other cases 

 dull and rough in its fracture, approaching to the rocks called Trachyte 

 and Claystone. Its colours are usually dull-gray, reddish-brown, and 

 greenish. It often contains scattered distinct crystals of lighter 

 coloured felspar, and is then Felspar Porphyry. 



Chranite, composed of distinct crystals of quartz, felspar, and mica. 

 Granite is less abundant in this district than in the Lower Silurian 

 area, next to be noticed, and the varieties which occur are often less 

 perfectly crystalline, or have a less resisting felspar, causing them to 

 decompose readily. 



Syenite consists of distinct crystals of reddish, gray, or white felspar, 

 with a smaller quantity of dark-coloured hornblende and some quartz 

 — the whole forming a material somewhat similar to granite, with 

 which it is often confounded. We may associate with this rock, or 

 with greenstone and compact felspar, a number of nameless rocks in 

 which crystalline felspar forms the chief ingredient, with or without 

 quartz and hornblende. 



