l6 NEW YORK STATE MUSEUM 



abundance. Other species occur in the Rochester shale and a few 

 species extend into the Devonic. The group however reaches its 

 culmination in the lower formations of the State and when found 

 they are nearly always seen to occur in thin black bands of shale 

 similar to that of the Clinton. 



The graptolite shales are well shown in Palmer's glen near Brighton. 

 Here the shales appear well down in the glen where the two streams 

 unite. Loose fragments of dark shale covered with graptolites may 

 be found in the stream bed, while the bank of the stream affords a 

 very favorable place for collecting. 



l7'ondequoit limestone. This limestone constitutes the highest mem- 

 ber of the Clinton group in this section. The name is from the town 

 of Irondequoit, which lies between Irondequoit bay and the Genesee' 

 river. 



The limestone rests directly on the Williamson shale, and its 

 numerous layers of limestone are separated by bands of shale of 

 various thickness. . In the lower portion of the formation, the shale 

 is similar to the green shale of the Williamson member, but in the 

 upper part the shale is gray and resembles that of the Rochester 

 formation. Some of the layers of limestone show a crystalline struc- 

 ture, the rock being made up largely of the broken remains of corals, 

 crinoids and various shells. The limestone is quite variable in com- 

 position, and from the presence of iron pyrites it is often stained 

 with iron as a result of the decomposition of the pyrites. 



Of special interest is the occurrence of numerous reeflike struc- 

 tures at the top of this limestone. These are mostly lenticular in 

 shape and several are found in the outcrops on the Genesee river. 

 These structures are very numerous and can be observed at many 

 points where the Irondequoit limestones outcrop, from Wayne 

 county to the Niagara river. 



A special study of these reef structures has been made by C. J. 

 Sarle,^ who has shown that they consist mainly of masses of bryo- 

 zoans which are still in position as when formed. The rapid growth 

 of these masses is indicated by the fact that in some cases they more 

 than kept pace with the accumulating sediment and the mass rises 

 above the general level. The growth of these masses finally ceased 



I Amer. Geol. Nov. 1901. p. 282. See also Clarke. N. Y. State Paleontol. An. Rep't. igoi. 

 p. 428-31. 



