192 NEW YORK STATE MUSEUM 



of these older joint channels (for our first conclusion asserts that 

 the present ones are all new) we must further conclude that the 

 Wisconsin till sheet cut away the tops of these isolated limestone 

 hills to a depth even greater than the 20 or m.ore meters now occu- 

 pied by the existing system of joint channels. Now this third con- 

 clusion is one that can not be maintained and with it the first also 

 falls. There seems to be good and abundant evidence that these 

 hill tops were cut away by recent glacial action to a depth that 

 rarely exceeded 2 meters, and even though we should allow more 

 than five times that amount vv^e should still expect to find the 

 widened bases of the joint channels of the older period. Joint 

 solution through the action of precipitated waters is an exceediugly 

 slow process, and all that post-Hochelagan precipitation has been 

 able to accomplish is a washing out of sediments accumulated dur- 

 ing the recent period of submergence, and a very slight enlarge- 

 ment of the older channels. As the shore line caves are for the 

 greater part but modifications of these old joint channels the pre- 

 glacial origin of the one implies also the preglacial beginnings of 

 the other. 



The varied and cumulative evidence here presented seems to 

 the writer to amply warrant the conclusion that both joint channels 

 and caves are, in their larger features, preglacial. A few words 

 concerning their origin may assist in more clearly comprehending 

 the questions involved. 



Origin and development 



Where locally elevated regions have been unable to maintain a 

 m.antle of soil the surface rock is subjected to more rapid changes 

 and greater extremes of temperature than where such a mantle has 

 been allowed to accumulate. The effect of such changes tends not 

 only to develop and maintain joint crevices, but it opens them at 

 the end of the winter's cooling more widely and to greater depths 

 than can occur in covered regions. At or soon after the time of 

 widest and deepest opening the waters from melting snows and 

 spring rains enter with great freedom, and after passing rather 

 rapidly through them exit at lower levels thus quickly emptying 

 all spaces not kept full by capillary attraction. These spring waters 

 with a temperature of nearly zero degrees would contain more 

 abundant carbon dioxid than summer waters, for the snow mantle 

 of winter absorbs this gas not only from the air above but from 

 the fracture zone below. The period of widest open crevices would 



