86 



deeply opened joints are to be found in the Salem. The joints run 

 nearly east and west and north and south. In other words, one set runs 

 with the dip, and the other with the strike. The dip joints are the most 

 conspicuous. 



The weathering of these limestones does not differ in essential fea- 

 tures from that of limestones in general, except as it is influenced by 

 local conditions of temperature, rainfall and drainage, and by the ex- 

 ceptional purity of the rocks. It is to be expected that a nearly pure 

 carbonate of lime, in a region of rather copious rainfall and mild climate 

 would weather almost entirely by solution and other chemical processes, 

 rather than by mechanical processes. The limestones in question exhibit 

 the effects of solution on such an extensive scale as to warrant calling 

 particular attention to them; and it is for this reason that the present 

 paper has been prepared. To this end attention has been called to the 

 composition, texture and structure of these rocks, even at the expense 

 of repeating descriptions already many times recorded in the literature 

 of Indiana geology. It is only by understanding the intrinsic nature of 

 a rock that we can correctly appreciate and explain its metainorphism, 

 whether it be in the zone of weathering or in the deeper zones. 



The chief agent of weathering in the present case is meteoric water 

 charged with CO. and with organic acids (humic acids). The normal 

 annual rainfall in the region under consideration is 42 inches (somewhat 

 more in the southern counties), rather evenly distributed throughout the 

 year. The largest average precipitation has been in the month of July, 

 while the minimum has been in the fall months — September, October, 

 November. The mean annual temperature is 52° F. The topography of 

 the limestone region excepting its eastern and western borders is undu- 

 lating, and of rather mild relief. Rolling uplands in which the larger 

 streams are rather deeply intrenched are the characteristic features. The 

 conditions are therefore such as to admit of a comparatively copious 

 entrance of water into the rock and free egress at lower levels into the 

 main streams. Such conditions favor solution. Solution has also been 

 favored in the past by the heavily forested condition of the region before 

 its settlement by the white race. 



The water which finds its way to lower levels in the rock than can 

 be tapped by the local drainage is frequently returned to the surface 

 along joint planes in the deep valleys on the western border of the region. 

 A notable instance of this is the French Lick Valley, which must derive 



