AMERICAN GEOLOGY EAToNlAN BRA, 1820—1829. 265 



left for others to decide. He, however, thought it probable that it 

 operated from beneath, "and that, after it had opened for itself a 

 vent, and escaped through the rift caused by its action, the rock 

 strata of the western part fell into the cavity which had previously 

 contained the imprisoned agent." 



In 1S23 Dr. John J. Bigsby, a surgeon of the British army sta- 

 tioned in Canada, read before the Geological Society of London a 

 paper, subsequently published in their transactions, on the Geology 

 and Geography of Lake Huron. He noted that the 

 canada' s i^2°3 rk '" rocks of the north shore of the lake were mainly primi- 

 tive -granite, gneiss, basanite — quartz rock, conglom- 

 erate, and greenstone. The other shores were described as occupied 

 by secondary rocks, frequently fossiliferous, which he regarded as 

 forming part of an immense basin, which, "extending probably 

 without interruption from the southern shore of Lake Winnipeg, 

 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 

 Mississippi." On Thessalon Island was found a new species of Orth<>- 

 ceratite, which, though described and figured, was not named. Drum- 

 mond Island yielded also corals and a trilobite — Asaphus platycephalus. 

 Naturally no attempt was made to determine the relative age of the 

 rocks by means of these fossils. 



In the year following. Doctor Bigsby had an article, in the Annals 

 of the Lyceum of Natural History (New York) on the geology of 

 Montreal. He described the lowest rock of the region as a trap of a 

 kind unique in the Canadas, and illustrating "in a 

 ffMont reaS he i8 e 24 0gy beautiful manner the affinity existing between the 

 formation of which it is a member and the primitive 

 class in general." 



A horizontal shell limestone of a bluish-black color he noted as 

 forming the floor of the plain surrounding the hill on which the city 

 stands, and through this hill, as a center, the passage in all directions 

 of a large number of trap dikes. 



The sandstone of St. Ann's he rightly described as underlying the 

 limestone, and noted the presence of fossil Li/ngulse,, Terebratulse, 

 Trilobites, and Orthoceratites. 



In the consideration of the sands and gravels constituting the so- 

 called alluvial, he followed the trend of opinion of his time, regarding 

 them as products of the vast inland seas which succeeded the deluge. 



The coastal plain of the Eastern United States, which up to this 

 time had been studied in detail only by Maclure and II. H. Hayden, 

 was in 1823 made the subject of a special essay by John Finch, then 



