248 Field Columbian Museum — Geology, Vol. i. 



Water flowing down vertical joint planes usually produces pits with 

 walls of angular contour, of which the "Bottomless Pit" in Mam- 

 moth Cave may serve as a type. It is possible that the circular 

 contours may arise from a solvent action added in an unusual 

 degree to the erosive action of water. By this means the solid 

 angles of the limestone blocks formed by the junction of several ver- 

 tical with one horizontal joint plane might be dissolved away until a 

 dome-shaped cavity was formed, or the form may be due to a 

 concretionary structure of the limestone like that recently noted in 

 Idaho.* The consecutive removal of the centers of successive domes 

 would cause each to fall in turn, maintaining the dome-like shape. 

 Stream erosion on the floor of such halls may remove this rocky 

 debris as fast as it falls as has been the case at Helen's Dome, or the 

 rise of the conical pile of rocky debris (such as that known as 

 "Monument Mountain" in Rothrock's Cathedral), may nearly keep 

 pace with the fall of the domes above. It is evident that if this 

 process of caving in is continued until the surface is reached, 

 "cistern-like pits leading down into the bowels of the earth" will 

 be seen from above. Such is the description given by W. H.Holmest 

 of the cenotes or sacred wells seen in Yucatan, some of which are so 

 round and even-walled as to be taken for works of art. They are 

 often, Holmes states, 100 feet or more in depth and 200 or 300 feet 

 in diameter. It seems evident from what has been stated above that 

 human agencies need not be appealed to for the formation of such wells. 



Fissure Systems. — Systems of fissures forming rectangles or 

 parallelograms closely resembling those produced byDaubr£e's well- 

 known experiment illustrating the formation of joints by torsion are 

 to be seen at many places along the roof of the cave. As an exhibi- 

 tion of jointed structure on a horizontal plane they are very satis- 

 factory. Often a secondary system of fissures appears in conjunction* 

 with the primary one. In many places, such as the "Pillared Pal- 

 ace," the formation of stalactites and stalagmites has taken place 

 along the lines of the joint planes. The stalactites and stalagmites 

 extend, therefore, in straight lines in most cases directly beneath the 

 crevice made by the joint plane. 



Distribution of Bats. — Bats were found in all parts of the cave 

 which I entered, even in the so-called "Unexplored Regions," the 

 entrance to which is a passage averaging about one foot in height for 

 a distance of 60 feet. If the bats were especially numerous any- 

 where, it was in the hall known as the "Senate Chamber," which, 



*A curious mineral formation in Idaho Engineering and Mining Journal, March 2, 1901. 

 tField Columbian Museum Publication 8, p. 19. 



