34 On the Geography and Geology of Lake Superior. 



eluded between Cape Verd and Point Marmoaze, is the seat of 

 older formations. These are sienite, chiefly abounding about 

 Peek Island ; and greenstone, more or less chloritic, interstratified 

 with conglomerates, and alternating five times with vast beds of 

 granite, which some might pronounce a sienite from the abundance 

 of its hornblende. The stratification of the greenstone is usually 

 very indistinct, but often again it is clearly marked. The direction 

 is always east or east by north, with a northern or perpendicular 

 dip, except at the Dog River, where the former decidedly varies 

 from WNW. to north, with a perpendicular or easterly dip. 



It is important to note the great quantity of the older shell 

 limestone strewn in rolled masses on the beaches from Point 

 Marmoaze to Grand Portage. It has not yet been seen in situ. 

 From the crags of Michipicoton to the latter place it forms a con- 

 siderable item in the amount of the debris. Its organic remains are 

 trilobites, orthoceratites, encrinites, productte, madrepores, terebra- 

 tulse, 8(c. I met with a large loose mass of pitchstone porphyry 

 at the west angle of Michipicoton Bay, the opposite angle being 

 trappose. 



I have now to describe with some minuteness, the nature, con- 

 tents, and connexion of the rocks above enumerated, as they occur 

 in succession on the north shore ; and commencing from Gros Cap. 

 This concluded, I shall give a rapid view of the principal points in 

 the geology of the south shore. 



[To be continued.] 



AuT. II. Of the Effects of the Induced Magnetism of an 

 Iron Shell, on the Rates of Chronometers. By George 

 Harvey, Esq. F.R.S.E. M.G.S., 8^c. 



[Communicated by the Author.] 

 The differences known to exist between the sea and land-rates 

 of chronometers, have led to many interesting inquiries respect- 

 ing the influence of magnetism, on these delicate and valuable 

 machines. The investigation has been contemplated generally 

 under two points of view ; first, as regards the effects of permanent 



