182 EARTH FEATURES AND THEIR MEANING 



such crevices that in the course of time the mineral matter carried 

 in solution by the water is deposited to produce the ore veins 

 and the associated crystallized minerals. 



The caverns of limestones. Where limestone formations have 

 a nearly flat upper surface, a large part of the surface water enters 

 the rock by way of the joint spaces, which it soon widens by solu- 

 tion into broad crevices with well-rounded shoulders. At joint 

 intersections solution of the limestone is so favored that the water 

 may here descend in a sort of vertical shaft until it meets a bedding 

 plane extending laterally and offering more favorable conditions 

 for corrosion. Its journey now begins in a lateral direction, and 

 solution of the rock continuing, a tunnel may be etched out and 

 extended until another joint is encountered which is favorable to 

 its further descent into the formation. By this process on alter- 

 nating shafts and galleries the water descends to near the surface 

 of the water table by a series of steps, and is eventually discharged 

 into the river system of the district (Fig. 191). Within the larger 



caverns the water at the lowest level 

 usually flows as a subterranean river 

 to emerge later into the light from be- 

 neath a rock arch. 



FIG. 191. -Diagram to show the Fr m the P lan of a System of COn- 



reiation of caverns in limestone necting caverns it may often be ob- 

 to the river system of the die- serve d tha t the galleries of the several 



tnct and to the "swallow ., , ... .. 



holes "upon the surface. levels are alike directed along two 



rectangular directions which indicate 



the master joint directions within the limestone formation. This 

 is especially clear from the map of the galleries in the explored 

 portions of the Mammoth Cave (Fig. 192). 



Swallow holes and limestone sinks. Above the caverns of 

 limestone formations there are selected points where the water 

 has descended in the largest volume, and here funnel-shaped 

 depressions have been dissolved out from the surface of the rock. 

 In different districts such depressions have become known as 

 " sinks," " swallow holes," entonn&irs, and Orgeln. Wherever the 

 depressions have a characteristic circular outline, there can be 

 little doubt that they are the product of solution by the descend- 

 ing water, and have relatively small connections only with the 

 subterranean caverns. They have thus naturally collected upon 



