SHELL COLONIES 447 



Shropshire, and, according to the observations of the author of this 

 volume, are in all essentials similar to those of the New York 

 Siluric. They form a marked contrast to the surrounding rock, 

 Avhich is generally a well-bedded calcarenyte, not only by their struc- 

 tureless form, but by the compactness and fineness of the material 

 composing them. The foundation of those balls appears to be 

 formed by a branching coral, while other organisms seem to be 

 rare. 



STRUCTURES FORMED BY THE GROWTH OF SHELL 



COLONIES. 



Tepee Buttes. Gilbert and Gulliver have described a series of 

 buttes or erosion hills of conical outline carved from the Cretacic 

 Pierre shales along the Arkansas River and some of its tributaries 

 in Colorado. (Gilbert and Gulliver-38.) From their resemblance 



Fig. g8. View of a group of Tepee-Buttes near Canyon City, Colorado. 

 (After Gilbert and Gulliver.) 



to the Indian "tepee" or conical hut, they have applied the name of 

 tepee buttes to them. (Figs. 98, 99.) The buttes have an irregularly 

 cylindrical core of limestone, varying from 2 to 24 feet in diameter 

 in different examples. The average diameter is, however, 10 to 15 

 feet, while the height is unknown. (Fig. 100.) This limestone core 

 consists of an aggregation of shells, "embedded in a matrix which is 

 composed of fragments of shell, water- worn grains of calcite, foram- 

 inifera and clay." The shells are mostly those of Lucina occi- 

 dentalis var. ventricosa, which makes up by far the larger part of the 

 core. With these occur other characteristic marine Cretacic species. 

 The core is enclosed in the Pierre shales, the passage from the 

 limestone to the shale being abrupt. There is, however, a certain 



