Dr. BiGSBY on the Geography and Geology of Lake Huron. 191 



north-west; and it dips at a high ang-le to the north-east^ when not absolutely 

 vertical. 



The greenstone of the large island close to Portlock Harbour varies much. 

 In one part it is nearly pure hornblende, splitting- into cubic blocks ; in an- 

 other it gradually resumes its conglomerated state, the nodules being small 

 and rare. At the south-west end it is very slaty for a square mile. 



At the place where the hornblende abounds, thin waving veins of ligni- 

 form asbestus are common. The centre only of the vein is pure, the sides 

 passing into greenstone. Vertical seams of quartz, with drusy cavities of 

 quartz crystals, are often met with ; and thready veins of galena also. I found 

 a mass of this ore loose, on the opposite side of this channel, weighing one 

 pound and a half. 



The precipices and steeps of the main in the Narrows of Pelletau are also 

 greenstone ; but, as usual, of different aspects. The bluff at the lower end 

 is only slightly slaty. It contains a confused mass of quartz veins, with a 

 small quantity of copper pyrites, and the carbonate of that metal. The middle 

 portion of these cliffs is extremely splintery, and appears to be ferruginous. 

 At the head of the Narrows the greenstone is much less disintegrated, and 

 dips into the clear and deep waters in compact black walls. I have passed a 

 league into the interior from the Narrows and Channel of Pelletau, without 

 finding any remarkable difference in geological structure between the interior 

 and the shores. The contiguous islets are of a similar formation, and are 

 composed of aggregated ridges rising to a great height. 



These greenstones dip from the secondary strata on the south, in the same 

 manner as at Malbay, 90 miles below Quebec, gneiss and mica-slate dip from, 

 and abut against, a horizontal calcareous conglomerate full of organic remains, 

 and, among others, of three species of orthoceratite. 



Having now described, as far as my information extends, the primitive rocks 

 of Lake Huron, I proceed to notice the secondary rocks of the same district. 

 They are a portion of an immense basin, which, extending probably without 

 interruption from the southern shore of Lake Winipeg, spreads itself over the 

 greater part of Lakes Superior*, Huron, and Simcoe, the whole of Lakes 

 Michigan, Erie, and Ontario, much of the western part of the state of New 

 York, the whole of the states of Ohio, Illinois, Indiana, and Michigan, and 

 the rest of the valley of the Mississipi. In describing the southern boundary 

 of the primitive chain, I have already traced the northern and eastern limits 



* Dr. Wright, Inspector of Hospitals, has a specimen of chalk from the neighbourhood of 

 Lake Superior. 



VOL. VI. 2c 



