38 The Middle Devoxiax Deposits of Maryland 



There is also structural evidence iu Xew York, as reported by Dr. 

 J. M. Clarke, favoring the reference of the Helderbergian series to the 

 Devonian. Dr. Clarke says : " The fauna of the Tentaculite limestone 

 [Manlius] makes its first appearance not far above the gypsum beds, and 

 in its most perfect development it is clearly an uppermost Siluric fauna, 

 having only very remote relations with the fauna of the remaining divi- 

 sions of the Helderbergian group in eastern ISTew York. The inference 

 that the upper limit of the Siluric system is properly to be placed at the 

 top of the Tentaculite limestone is corroborated by stratigraphic structure 

 which shows in places distinct unconformity between the Tentaculite lime- 

 stone and the overlying strata." ^ 



In 1855, Professor Dana in his address as President of the American 

 Association for the Advancement of Science proposed divisions of geologic 

 time subordinate to the great ages which were Silurian, Devonian, Car- 

 boniferous, et cetera; stating that these subordinate divisions would 

 depend " On revolutions in the earth's surface, marked bv abrupt trans- 

 itions either in the organic remains of the region, or in the succession of 

 rocks. Such divisions are not universal. Each continent has its own 

 periods and epochs."" Dana called the first subdivision of the ages, 

 periods and these were again divided into epochs, an epoch corresponding 

 in general to that division of the rocks which had been called a group 

 by the iSTcw York geologists in their final reports. So the Devonian 

 or Age of Fishes was divided by Dana into the Upper Helderberg, 

 Hamilton, Chemung, and Catskill periods.' This classification appear- 

 ing in greater detail in the first edition of his Manual of Geolog}- pub- 

 lished in 1863, has been amplified in the three succeeding editions but 

 its essential characters have not been changed. In the last edition the 

 Devonian was divided into the Oriskany, Corniferous, Hamilton, and 

 Chemung* periods. This was, however, only a minor change du-.; to a 

 better knowledge of the formations, the Upper Helderberg period being 



^N. Y. State Museum. .53d An. Rep. of the Regents 1899, Vol. 1, 1901, p. 671. 



" Proc. Am. Assoc. Adv. Science, VoL 9, 1856, p. 5. 



'75M., p. 14. 



"Man. Geol., 4th ed., 1895, p. 576. 



