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



of quartz^ of greenstone, or of the red ingredient found in the rock of Le 

 Serpent. This latter conglomerate has been noticed as occurring in situ in 

 the Channel of Pelletau. 



It is only about the Narrows of Pelletau that the rock of Le Serpent has 

 been seen in a rolled state. Breccias similar to the conglomerates are not un- 

 common ; but I have never found them in situ. 



Pebbles of red sandstone, and quartzose or slaty limestone, have a very li- 

 mited range : they only nov\f and then wander as far as the Manitoulines, the 

 southern shore, or Michilimackinac. 



I have already noticed the quantity of primitive boulders found on the Isle 

 of St. Joseph. The beach of the rivers Thessalon and Missassaga is covered 

 by boulders of black ti'ap, granite, gneiss, and jasper-conglomerate. 



The Georgian or Penetanguishine arm of the lake is loaded to excess 

 with sand and rolled pebbles. Penetanguishine, and much of the south-east 

 coast of this arm of the lake, is a collection of sand-hills, enveloping quartzose, 

 granitic and amphibolic blocks of all sizes, and in vast quantity. 



Passing into the southern division of the lake, 64 miles south of Cabot's 

 Head, the limestone chffs of the Manitouline range are succeeded by chffs of 

 clay. Prom this point beds of clay, covered towards the upper part of the 

 river St. Clair by thick beds of sand, extend for 150 miles to Lake Erie, and 

 thence along the northern shore, which presents a series of clay cliffs and 

 sand-hills, to the north-eastern extremity of the lake. The whole of the in- 

 tervening shores and woods are strown with rolled blocks of gneiss, porphyry, 

 conglomerate, and greenstone, such as prevail on the northern shore of Lake 

 Huron. In a south-westerly direction, the clay-beds prevail over the Michi- 

 gan territory, and the states of Indiana and Illinois, to an unknown distance. 

 In the two last-mentioned states (which I have not visited) rolled blocks 

 abound. 



The argillaceous and sandy banks of the southern shore of Lake Huron 

 are conspicuous near Point aux Barques, in the Gulf of Saguina, and about 

 Presquisle. The debris of the rocks of the northern shore are here rare, and 

 much rolled. — Staurotide was picked up on the southern shore by Mr. School- 

 craft. 



Besides the sand and boulders before spoken of, which are ancient, and 

 have travelled from a distance, there are fragments of another character, which 

 may be called native, reposing on the parent rock, or not far removed from it. 

 This debris is comparatively recent, having been detached by various natural 

 causes, such as torrents, change of temperature, &c. The latter agent ope- 

 rates either by the expansion and contraction of the rock itself, or of the water 



