424 • PRINCIPLES OF STRATIGRAPHY 



The Onondaga Reefs of Neiv York. The Onondaga limestone 

 of the State of New York is a good example of an ancient 

 formation resulting from the growth of numerous coral reefs and 

 the accumulation of their attendant deposit of clastic lime sands. 

 (Grabau-40.) The formation varies in thickness from about 50 

 feet in the eastern part of the state to about 200 feet in the western, 

 and is in part continued westward into Ohio and Michigan by the 

 Columbus and Dundee limestones, the age of which, however, ap- 

 pears to be somewhat younger. Throughout this extent the forma- 

 tion rests upon an old land surface, or the deposits of continental 

 origin formed during that period, and partly reworked by the trans- 

 gressing sea. At first more or less siliceous limestones containing 

 shells of molluscs and brachiopods were formed (Schoharie grit, 

 Decewville beds), and on this foundation the corals began to 

 grow. It appears that the corals formed a series of barrier reefs 

 roughly parallel to the old coast line, which passed through New 

 Jersey, southeastern Pennsylvania, central Maryland and West Vir- 

 ginia. Close to the shore in the northern part clastic deposits, 

 chiefly grits, were forming, but the southern section is marked by 

 the accumulation of black muds (Romney shale) with a depau- 

 perate fauna of few species. The conditions were much the same 

 as we find now in Florida, where the inner lagoon is a region of 

 black mud deposits with a mingling of marine and fresh-water 

 organisms. In Pennsylvania and in New York a succession of 

 reefs was apparently formed parallel to this southeastern shore, 

 each series progressively farther removed to the north and west 

 from this coast. The gradual advance of the inner lagoon con- 

 ditions toward the northwest is shown by the progressive creeping 

 out of the black mud deposits over the old dead reefs, the Onondaga 

 limestone being progressively covered by the black Marcellus muds, 

 which are to a large extent the lagoon equivalents of the reef 

 limestone farther seaward. In most localities the deposits formed 

 immediately over the dead reefs continue to be of lime sands 

 derived, no doubt, from the still-living reefs farther out to sea. 

 These lime sands were, however, abundantly charged with silica, 

 probably in the form of sponge spicules and diatoms, and these 

 gave rise to the layers of chert found in these upper rocks in such 

 abundance in some localities, and on which account this rock early 

 received the name of Corniferous limestone. These corniferous de- 

 posits swell between the reefs, where most probably the lagoons 

 were situated, and they are buried in turn by the encroaching black 

 mud deposits which represent conditions of deposition unlike those 

 found in the open sea. That the deposits of black mud were pro- 



