€4 GEOLOGICAL FORMATIONS 



Above this bed is a slightly thicker member, of an intensely 

 white color. It is encrinital chiefly, like the other, but in ad- 

 dition contains a large amount of comminuted shell material 

 derived from molluscan remains. In places the shell fragments 

 predominate, forming a fine shell breccia, not unlike the well- 

 known coquina rock of Florida. It is, however, so compact 

 that good specimens of fossils are almost impossible to dis- 

 lodge. The layers of this bed contain also considerable chert 

 in small nodules and nodular bands. When first exposed in 

 quarrying, these cherts are very compact, translucent, and 

 break with a conchoidal fracture. Upon exposure these flints 

 quickly slacken like quicklime to a fine, intensely white pow- 

 der. If examined before the process of disintegration has 

 proceeded more than half way, the white nodules are found to 

 be charged with fossils, which, when taken out, cannot be told 

 from plaster-of-Paris casts. Before being affected by atmos- 

 pheric agencies, few or no traces of organic remains are to be 

 detected in the cherts. But they actually contain a very ex- 

 tensive assemblage of fossils, and in a perfect state of preser- 

 vation when collected at the right time. They afl'ord unusual 

 opportunities for both structural and systematic studies, for 

 many of the features commonly not met with are here found 

 beautifully preserved. Such are the internal characters of 

 crinoids and brachiopods, and the delicate ornamentation of 

 nearly all groups. 



Careful comparisons of the fossils from these cherts and 

 those of the surrounding limestones show that the forms to a 

 great extent are identical. Moreover, numerous shells and 

 crinoids are found partly imbedded in the chert and partly in 

 the limestone, with a sharp line of separation, indicating clearly 

 that the silicious impregnation was acquired long after the 

 original deposition of the beds, and was not due to a greater 

 silicity of the waters in which the calcareous deposits were 

 made, as has been held by many prominent writers. This is 

 in accordance with observations made elsewhere in the Bur- 

 lington limestone. 



