248 FreELD CoLuMBIAN MusrEuM—GEOLOGy, VOL. 1. 
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 beappealed to for the formation of such wells. 
FissurRE Systems.—Systems of fissures forming rectangles or 
parallelograms closely resembling those produced by Daubrée’s well- 
known experiment illustrating the formation of joints by torsion are 
to be seen at many places along the roof of the cave. Asan 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. 
+Field Columbian Museum Publication 8, p. 19. 
