APPENDIX. 317 



deposited originally in horizontal beds, its position has been dis- 

 turbed in many localities. 



Plastic Clay Stratum of the Lakes. — The northern ex- 

 tremity of Muddy Lake — a sheet of water some twenty miles in 

 length — is the head of the straits, and the beginning of the River 

 St. Mary's. This sheet of water has the property of being ren- 

 dered slightly whitish, or turbid, by continuous winds. Its bottom 

 appears to be formed of the same plastic blue clay which obstructs 

 the passage of vessels of large draft on the St. Clair flats, and 

 forms an impediment of a similar kind in this river in Lake 

 George. This stratum seems to be the result of causes not now 

 in operation. If dredged through, or excavated, there is no rea- 

 son to suppose it would again accumulate ; for the waters of the 

 lake are clear and pure, and carry down no deposit of the kind. 

 These clay deposits remain to attest physical changes which are 

 past. They denote the demolition of formations of slate in the 

 upper regions, which have been broken down and washed away 

 when the dominion of the waters was far more potential than they 

 now are. 



This formation is favorable to the growth of some species of 

 fresh-water shells. I observed several species of the anadonta 

 and the plenorbis, and think, from the broken valves, that re- 

 search would develop others, 



PORPHYEY AND CONGLOMERATE BOULDEES. — A formation of 



red jasper, in common white quartz, exists, in the bed of intersec- 

 tion, on the southeastern foot of Sugar Island. The fragments 

 of jasper are of a bright vermil red, quite opaque, and have pre- 

 served their angles. I had observed fragments of the formation 

 along the shores of the lower part of the straits, and even picked 

 up some specimens, entirely abraded, however, on the south 

 shores of the Huron, between the "White Rock and Michili- 

 mackinac — a proof of the course of the drift. 



The granitic conglomerates appear quite conclusive, one would 

 think, of the results of fusion. The attraction of aggregation 

 would seem inadequate to hold together such diverse masses. In 

 these curious and striking masses we see the red feldspathic 

 granite, black and shining hornblende rock, white fatty quartz, 

 and striped jasper, held together as firmly, and polished by attri- 

 tion as completely, as if they were — what they are not — the re- 

 sults of crystallization in this aggregate form. 



