J. D. Dana on American Geological History. 313 



This key soon opened to us a knowledge of New England 

 geology, mainly through the labors of Prof. Hall, and also of Pro- 

 fessor H. D. Eogers, following np the survey of President Hitch- 

 cock; 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 Carbon- 

 iferous of New York and Pennsylvania.* 



Let us now briefly review the succession of epochs in American 

 geological history. 



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

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

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

 left as dry land, which appear to have remained so, as a general 

 thing, in after times ; for no subsequent strata cover them. Such 



of Michigan, G. Troost and lately J. M. Safkord of Tennessee, J. Greene, J. Locke, 

 C. Whittlesey, I. A. Lapham, G. C. Swallow, J. G. Norwood, B. F. Shumard, be- 

 sides the investigators in Canada, Sir W. E. Logan, J. Bigsby, J. W. Dawson, 

 T. S. Hunt and others. 



The Carboniferous formation was early studied in many of its details by Dr. S. P. 

 Hildreth. But the successive strata of the whole formation from the Devonian 

 through the Subcarboniferous and Coal Measures, were first systematized by the 

 Professors Rogers, though without yet marking out in any of their publications the 

 subdivisions of the coal measures themselves and the characteristic fossils of each, 

 as had been done for the Devonian and Silurian by the New York Geologists. Other 

 researches on the coal beds have been made by R. C. Taylor and J. P. Leslie in 

 Pennsylvania, J. Hall, D. D. Owen, and others in the states of the Mississippi valley, 

 J. S. Newberry on the fossil plants and fishes of the Ohio coal measures, Hitchcock 

 and C. T. Jackson on the coal beds of Rhode Island ; Dawson, Lyell, Jackson, etc., 

 on the New Brunswick and Nova Scotia beds ; Lea, Wyman, Leidy, Lyell and 

 Dawson on Reptilian and other carboniferous fossils. 



The parallelism of the rock formations of the east and west has been determined 

 mainly through the researches of Prof. Hall, who first presented his views on the 

 subject in 1841. 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 Triassic or Jurassic period, which 

 occur in the Connecticut valley and in other valleys parallel with the Atlantic bor- 

 der to the south, and also to the north beyond Nova Scotia, have been specially 

 investigated by D. Olmsted, E. Hitchcock, J. G. Percival, Professors Rogers, E. Em- 

 mons, J. W. Dawson, C. T. Jackson, F. Alger; and as regards the vertebrate fos- 

 sils, by E. Hitchcock, J. Deane, W. C. Redfield, J. H. Redfield, J. Wyman, J. Leidy, 

 I. Lea, and Prof. Owen of London ; and the plants, by the Professors Rogers, 

 C. T. F. Bunbury, and E. Hitchcock, Jr. 



* The labors of Sir W. E. Logan have thrown great light upon New England 

 geology, and are giving a defmiteness to our knowledge hitherto unattained. 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 north- 

 ern extremity as well as in New Jersey ; the calcareous mica slate of western Ver- 

 mont, has been shown to be Upper Silurian in age, it being uncrystalline limestone 

 towards Gaspe, partially metamorphic and still containing 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 this subject. 



SECOND SERIES, VOL. XXII, NO. 66. — NOV., 1856. 



40 



