448 



PRINCIPLES OF STRATIGRAPHY 



amount of interpenetration of the limestone and shale. "Processes 

 of the limestone embrace portions of the shale, and the contiguous 

 shale contains outlying lumps of the limestone." The best exposed 

 cores have a stratified appearance, with beds from one to three feet 

 in thickness, and separated by more or less continuous horizontal 



Fig. 99. A single tepee-butte, showing the top of the limestone core. (After 

 Gilbert and Gulliver.) 



partings of shale. The presence of these shale partings indicates 

 that the core was not built any faster than the surrounding de- 

 posits of mud. 



Various explanations have been offered by the authors cited for 

 the origin of these cores. The best of these, and the one favored 

 by the authors, regards these accumulations of shells as colonies, 



Fig. 100. Ideal section of a tepee-butte, showing the core of organic lime- 

 stone, the enclosing bedded shales, and the cover of talus. (After 

 Gilbert and Gulliver.) 



which, having begun in a given place, continued there through a 

 long period of deposition, the later organisms living on the shell 

 heaps of their dead predecessors without migrating much outward 

 onto the surrounding muddy ocean bottom. Professor Shaler has 

 suggested that Lucina may have had a byssus, by means of which 

 it would attach itself to the dead shells of earlier genera- 

 tions, which would readily explain the localization of these deposits. 

 On these shell heaps would be the best feeding ground for other 



