AMERICAN GEOLOGY DECADE OF L830— 1839. 313 



coal measures, and the New Red sandstone, and in the ^ixth group, 

 Eocene Tertiary, diluvium, and alluvium. 



Hitchcock described the Connecticut as flowing through a synclinal 

 valley, a valley which became nearly filled by the red sandstone deposit. 

 This last, he thought, had since been eroded away, so that the valley 

 as it now exists may "in some sense" be considered "as a valley of 

 denudation." Most of the valleys in Massachusetts he still regarded 

 as valleys due "to the elevation and dislocation of the strata." 



He noted that the animal remains found in the older rocks differed 

 the most radically from the existing species, and also that the organic 

 remains found in northern portions of the globe corresponded more 

 nearly to existing tropical species than did those now living in the 

 same localities. Further than this, he announced that different species, 

 genera, and families of animals began their existence at very different 

 epochs in the earth's history, and that the same species rarely extended 

 over from one formation into another. Notwithstanding this, though 

 giving lists of fossils found in rocks of various horizons, he made' no 

 attempt to determine their relative ages by means of them and regarded 

 correlation by such aids as impossible. Following Phillips and Lyell 

 in many of his statements, he yet announced as a principle that " Rocks 

 agreeing in their fossil contents may not have been contemporaneous 

 in their deposition," although they might not differ greatly in age. 

 '•From all that has been advanced," lie wrote, "it appears that an 

 identity of organic remains is not alone sufficient to prove a complete 

 chronological identity of the rocks widely separated from each other; 

 but it will show an approximate identity as to the period of their 

 deposition, and in regard to rocks in a limited district it will show 

 complete identity. 1 ' Where both the mineral nature and the character 

 of fossils were identical, identity or synchronism would be much more 

 probable, but a want of such agreement, so far as it related to mineral 

 character, was not regarded as fatal to the idea of synchronism. 



He argued against the idea that the stratified primary rocks are 

 merely the detrital or fossiliferous rocks altered by heat, but regarded 

 them rather as products of both mechanical and chemical action by 

 aqueous and igneous agencies when the temperature of the crust was 

 very high and before organic beings could live upon it. 



He noted that the dolomitic rocks seemed genetically related to the 

 limestones, but his ideas as to the methods by which the changes had 

 been brought about were naturally somewhat crude when considered 

 in the light of to-day. From a study of the field relations of the dolo- 

 mites of Berkshire County and other localities he came to the conclu- 

 sion "that all the cases of dolomitization in Massachusetts occur either 

 in the vicinity of a fault, or of unstratiried rocks, or in the midst of 

 gneiss, where the evidences of the powerful action of heat in the 

 induration of the limestone and the obliteration of its stratification is 



