FOSSIL REEFS: DEVONTC 425 



gressively replacing the deposits of limestone is shown by the fact 

 that the highest layer of these rocks in western New York, the 

 Agoniatite limestone, extended eastward during a temporary east- 

 ward transgression of the sea, and rests on 50 feet of these black- 

 muds in eastern New York, where the Onondaga is only 50 feet 

 tiiick, and on 175 feet of black muds in Maryland, where the Onon- 

 daga limestone is absent. In western New York, where the Onon- 

 daga is about 200 feet thick, it consists of reefs in the lower part, 

 followed by shell limestones and cherty beds in the upper. This in 

 turn is followed by over 50 feet of black shale practically barren 

 of fossils except in a few localities, and there sometimes including 

 the remains of eurypterids and other arthropods adapted to brack- 

 ish or fresh waters and rarely found except in the deposits of shal- 

 low coastal lagoons, protected embayments or fresh-water lakes or 

 rivers. In Ohio and southern Michigan the deposits of limestones 

 of the Onondaga type began much later, and also continued much 



Fig. 86. Diagram showing successive advances of the coral reefs and accom- 

 panying clastic lime sands and the covering muds of the inner 

 lagoons. 



later, so that the overwhelming black muds reached that part of the 

 sea only toward the close of the deposition of black Marcellus muds 

 in western New York. (Fig. 86.) 



Few of the actual reefs have been located and described, this 

 being due more to lack of attention directed toward their structure 

 than to their non-exposure. It is also important to bear in mind 

 that modern quarrying operations are carried on in the bedded 

 strata of inter-reef origin, and that the structureless reefs, so diffi- 

 cult to operate in, are avoided. This is strikingly shown by the 

 quarries opened in these limestones in* western New York (Wil- 

 liamsville), two of which are found on opposite flanks of a reef 

 several hundred feet in diameter ; the reef itself being untouched. 

 (Grabau-4o:j./0.) In the quarries the various dips away from the 

 reef are shown, these, however, not often exceeding 10°. (Fig. 87.) 

 On one side the sloping floor .of the quarry is formed by the 

 upper surface of the reef, and here the composition of the reef 

 can be ascertained. Enormous heads of branching Syringoporas, 



