xlviii GENERAL SUMAIARY OF SCIENTIFIC AND 



nortli and east rests directly upon the rocks of the second 

 I'aiina, and even upon the older crystalline rocks (see the Jiec- 

 ord for 1872, p. xxxv-xxxvii). Hunt, in this connection, calls 

 attention to the absence over New England and the British 

 maritime provinces of the second or Trenton fauna, and, ex- 

 cept to the far eastward, of the third or Medina-Niagara 

 fauna. The Lower Helderberg, corresponding to the Ludlow 

 rocks of ]]ritish geologists, really includes a fourth paleozoic 

 launa, and is, the upper division of the true Silurian, while 

 the Medina-Niagara group is the lower division ; the interval 

 between the two presenting a great paleontological and chron- 

 ological break, which is marked in central New York and 

 western Ontario by the deposition of the dolomites, salt, and 

 gypsum of the Salina formation. 



Professor Newberry, in discussing the often-observed cy- 

 cles of deposition in stratified rocks, has pointed out that 

 the invasion of the sea, resulting from a subsiding continent, 

 produces first a sheet of sea-beach sand and gravel, followed 

 by oif-shore deposits. To these succeed the limestones de- 

 posited in the open sea, which are followed by the mixed sed- 

 iments of shaly or earthy limestones, the product of the re- 

 treating sea, completing the cycle of deposition ; after which 

 ages may elapse before a second submergence permits the 

 deposition of a new series, characterized by a new fauna and 

 a new flora. In this connection Hunt has insisted upon the 

 importance of the deposits from evaporating inland seas, 

 marked by magnesian limestones, often %vith gypsum and 

 salt, and in some cases destitute of animal life, which, as in 

 the case between the Niagara and the Lower Helderberg, 

 gave rise to a paleontological break. 



The phenomena of the disintegration of crystalline rocks, 

 as seen in the Blue Ridge, have been discussed by Dr. Hunt. 

 The process was one of chemical change, resulting in the de- 

 composition of the feldspars and hornblende, removing the al- 

 kalies, lime, and magnesia, and a portion of silica, and con- 

 verting the rocks, to a depth in some cases of a hundred feet or 

 more, into a soft, reddish clay ; in which the unchanged veins 

 and layers of quartz still remain to show the liighly inclined 

 position of the strata. This action took place under the in- 

 fluence of a highly carbonated and moist atmosphere, and 

 "was already at work in early paleozoic times. From the 



