24 



INTRODUCTION. 



largely made up of the entire or fragmentary shells of animals 

 belonging to this group. 



Still more important are the true Molluscs, the shells of which 

 have commonly been accumulated to form beds of limestone in all 

 the great periods from the Ordovician onwards. Some of these 

 " shell-limestones " are composed of the exuviae of sedentary Mol- 

 luscs, such as Oysters, and have therefore been formed in place. 

 Excellent examples of such limestones are afforded by portions of 

 the " Muschelkalk " of the Trias of Germany, and the massive 

 " Hippurite limestone " of southern Europe. In other cases, the 

 limestone is made up principally of the broken fragments of Mol- 

 luscan shells which have been subjected to the transporting and 

 abrading action of the sea ; and the rock has therefore been formed 

 in a manner similar to that in which the " shell-sand " of modern 

 shores has been produced. In still other cases, again, the limestone 

 has been formed by the slow accumulation on the sea-bottom of the 

 shells of "pelagic" Molluscs, which live at or near the surface of 

 the sea, and the skeletons of which fall to the bottom on the death 

 of their owners. Of this nature are the " Pteropodal ooze " of the 

 present period and the " Pteropodal limestones " associated with 



Fig. 8. — Section of a Pteropodal limestone, 

 made up of the shells of Styliolajissurella, 

 Hall, from the Devonian (Genesee Slates), 

 Canandaigua, United States. Enlarged 

 twenty times. (Original.) 



Fig. 9. — Section of a Tertiary limestone 

 (" Leitha-kalk "), from Nussdorf, near Vienna, 

 composed almost entirely of fragments _ of 

 Nullipores cemented together by a crystalline 

 matrix. Enlarged three times. (Original.) 



uplifted coral-reefs. Similar " Pteropodal limestones " are known to 

 occur even in deposits as old as the Devonian (fig. 8), but they are 

 of rare occurrence and are usually of small thickness. Lastly, it 

 is to be noticed that limestones may be formed as well by fresh- 



