On Ameriaxn Geological History. 405 



This key soon opened to us a knowledge of NewEno-land o-eo- 

 logy, mainly through the labors of Prof, Hall, and also of Profes- 

 sor H. D. Rogers, following up the survey of President Hitchcock* 

 and now the so-called primary rocks, granite, gneiss, schists and 

 crystalline limestones, once regarded as the oldest crystallizations 

 of a cooling globe, are confidently set down as for the most part 

 no older than the Silurian, Devonian, and Carboniferous of New 

 York and Pennsylvania. * • 



Let us new briefly review the succession of epochs in American 

 geological history. 



The Azoic Age ended, as was observed, in a period of extensive 

 metamorphic action and disturbance, — in other words in a vast 

 revolution. At its close, some parts of the continent were left as 

 dry land, wdiich appear to have remained so, as a general thino- 

 in after times ; for no subsequent strata cover them. Such are a 

 region in Northern New York, others about and beyond Lake Su- 

 perior, and a large territory stretching from Labrador westward 



The parallelism of the rock formations of the east and west has been de- 

 termined mainly through the researches of Prof. Hall, who first presented 

 his views on the subject in 18-tl, and continues still his investigations. The 

 examinations of De Verneuil; besides defining the limits of our Devonian 

 also contributed much on this subject. 



The red sandstone and trap regions of the Triassie or Jurassjic period, 

 which occur in the Connecticut valley and in other valleys parallel with the 

 Atlantic border to the south, and also to the north beyond N"ova Scotia, have 

 been specially investigated by D. Olmsted, E. Hitcucock, J. G. Percival, 

 Professors Rogers, E. Emmons, J. "W. Dawson, C. T. Jackson, F, Alger • 

 and as regards the vertebrate fossils, by E. Hitchcock, J. Deane, W. C 

 Redfield, J. H. Redfield, J. Wyman, J. Leidt, I. Lea, and Prof Owen of 

 London ; and the plants, by the Professors Rogers, C. T. F. Runbury, and 

 E. Hitchcock, Jr. 



* The labors of Sir "W. E. Logan have thrown great light upon New Eng- 

 land geology, and are giving a defiuiteness to our knowledge hitherto unat- 

 tained. He is finding that some of the crystalline New England rocks which 

 stretch north into Canada, are there uncrystalline and fossiliferous, and thus 

 is putting the question of age beyond doubt. The Berkshire limestone has 

 thus been determined at its northern extremity as well as in New Jersey ; 

 the calcareous mica slate of western Vermont, has been shown to be Upper 

 Silurian in age, it being uncrystalline limestone towards Gaspe, partially 

 metamorphic and still C(>ntaining distinct traces of fossils in the valleys of 

 the river St, Francois and Lake Memphremagog, and farther south becoming 

 more crystalline as well as calcareous and losing all indications of fossils. 

 Prof. T. S. Hunt of the Canada Survey, has brought other facts to bear on 

 ithis subject. 



