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



of the basin in question : its southern limits, as far as I am aware, are only to 

 be found in the Gulf of Mexico, with numerous interruptions, however, from 

 the older rocks in Pennsylvania, Virginia, Alabama, &c. On the west it is 

 bounded very irregularly by the primary districts that lie to the west of the 

 Mississipi (from which it does not deviate far), and to the south and Avest of 

 Lake Superior. The basin has been represented to extend as far westwards 

 as the rocky mountains ; but Mr. Schoolcraft has shown that the rugged coun- 

 try about the mouth of the Missouri consists of gneiss, with primitive lime- 

 stone intermixed ; and on further research, primary rocks will probably be 

 found still further southwards. 



The connexion of the secondary with the primary rocks of Lake Huron has 

 been very imperfectly examined ; and, in fact, is almost wholly concealed by 

 the thick vegetation of those islets where the contact of the two formations 

 does occur : or, in other cases, by the wide intervening tracts of water. 



I shall first notice those rocks, which, though not primary, possess less de- 

 cidedly a secondary character. 



About the river Thessalon, on the large island opposite to, but seven miles 

 distant from, its mouth, and in the insular groups of the lake, north of False 

 Detour, my friend Major Delafield (American Agent under the 6th and 7th 

 Articles of the Treaty of Ghent) observed a granular quartz, forming the 

 north points of the islands, and dipping north, at an angle of 45°. On one 

 isle it was remarked to run imperceptibly into the greenstone slate that lay 

 beneath it. In High-cliif Island the granular quartz forms a precipice 100 feet 

 high. On this island limestone containing orthoceratites is met with, which 

 appeared to Major Delafield to alternate with the quartz. This quartz rock 

 is frequently seamed with white quartz, of which blocks, containing much 

 chlorite earth, lie loose on the shores. It is always hard, minutely granular, 

 and now and then very crystalline. It contains no petrifactions. 



This rock extends westward as far as Green Island, and is then succeeded 

 by the greenstones on the north of the Channel of Pelletau. 



Immediately on passing into the Lower Basin discharging into the Narrows 

 of Pelletau, a quartz rock shows itself obscurely among the marshes about 

 St. Joseph ; but from the insular barrier to Lake George, it is abundant, and 

 has a north-west course, and a dip which is either vertical or not discernibly 

 otherwise. At the barrier it consists of minute grains of vitreous quartz, 

 cemented by the same substance, rather powdery, opaque, and white. It is 

 somewhat easily frangible. Its fissures are sometimes lined with brilliant red 

 quartz crystals. 



The islands on the north of the Upper Basin, about the Narrows of Pelle- 



