lxxiv GENEKAL SUMMARY OF SCIENTIFIC AND 



strata, which have been regarded as primal paleozoic, are, 

 according to him, decayed crystalline schists, probably of 

 Huronian age. The interstratitied layers of hydrous iron ore 

 and brown hematite which they contain are regarded by him 

 as a result of the oxydation of beds of pyrites in the original 

 schists. A similar bed of limonite is found in the decayed 

 strata at the Hoosac Tunnel ; and Professor C. U. Shepard 

 long ago pointed out that the brown hematite ores of North- 

 western Connecticut belonged to decayed crystalline schists, 

 and were probably derived from the alteration of pyrites. 



Dawson, from his recent studies of the flora of the Upper 

 Coal Measures of Prince Edward's Island and Eastern Nova 

 Scotia, shows that there is here a passage from the Carbonif- 

 erous to the Permian, thus bridging over the interval which 

 in Eastern North America has hitherto divided the Carbon- 

 iferous from the Trias, which in most cases rests unconform- 

 ably on our older crystalline rocks. Woodward proposes 

 anew to group together in one series, under the old name of 

 Poecilitic, proposed by Phillips, the strata from the coal to 

 the Rhaetic beds at the base of the lias, thus including Per- 

 mian and trias. 



The question as to the nature of glacial agency, and to the 

 part formerly played by land-ice, continues to occupy the at- 

 tention of a large number of geologists. Bell, from his late 

 observations in Nicaragua, supposes that, even in tropical 

 America, glaciers at one time came within 2000 feet of the 

 sea-level, and finds the evidence of their action in deposits of 

 sand, gravel, and supposed moraines, though admitting the 

 absence of scratches on the rocks. According to Mr. Tylor, 

 the polar ice-cap, as conceived by Agassiz, would contain so 

 much ice abstracted from the sea as to reduce the level of it 

 600 feet, while Mr. Bell, who supposes that the ice-cap would 

 be present simultaneously at the two poles, admits a lower- 

 ing of the sea equal to 1000 feet. Shaler, however, shows 

 that an ice-cap extending over both hemispheres from the 

 poles to 45, with an average thickness of one mile, would 

 reduce the sea's depth over half a mile. Regarding all the 

 great horizons of conglomerate rocks as periods of glaciation, 

 he indicates the following : The Lower Cambrian, the close 

 of the Cincinnati group, the beginning of the Coal period, the 

 Trias, besides probably three periods in the Tertiary. Re- 



