GUELPH FAUNA IN THE STATE OF NEW YORK I I 5 



We may note, first, that the dolomite which carries the chert nodules 

 of the upper horizon (upper Shelby and Rochester) is highly magnesian." 



It shows no stratification, is usually dark and so bituminous that it gives 

 off a strong petroleum odor when fresh or struck with the hammer. It is 

 for the most part granular, though compact and contains numerous white 

 silicious concretions in which the fossils are preserved. Outside of these 

 nodules fossils are rarely found except remains of Stromatopora, Halysites 

 and other corals. 



It is claimed by Walther and other writers and may be regarded a 

 matter of general acceptance that noncrystalline dolomites carrying so high 

 a percentage of magnesia as these are distinctly coraligenous. 3 Coral rocks 

 of later geologic age may show higher percentages of magnesia than this as 

 the amount apparently increases the longer the process of diagenesis con- 

 tinues ; in other calcareous deposits the content of magnesia is always small 

 and this is in correspondence with the fact that shell limestones now form- 

 ing are low in magnesian content. The skeleton of living corals actually 

 contains but relatively little magnesian salts (M adrepora muricata 

 2.4% and Isis 6.3$) but it is known that during diagenesis or the sum of the 

 little known processes by which a sediment is changed into rock, 3 the coral 



'One of the white chert nodules from the Nellis quarry, Rochester gave the following: 



SiO 74-973$ M g 4-366% 



Ca 5-6i3% CO.&HaO 9-"*% 



Fe & Al 68% (Analysis by P. N. Coupland) 



The dark dolomite from the same locality gave: MgO 20.95% or M gC0 3 44±%. The 

 lighter dolomite from the lower Shelby bed at Shelby gave: MgO 16.43% or MgC0 3 36*%. 

 (Analyses by G. I. Finlay) 



•Walther. Einleitung in die Geologie als historische Wissenschaft. 1894. p. 663 etseq. 



^Doelter and Hoernes have supposed [Jahrb. k. k. geol.-Reichsanst. Vienna. 1875. 

 p. 331] that the magnesian salts of the sea water, specially Mg CI, act on the calcareous 

 secretions of the organisms as soon as formed. Walther [op. cit. p. 708] thinks the pro- 

 duct due in large measure to bacterial action, just as bacteria have been shown to pro- 

 duce deposits of calcium carbonate by forming ammonium carbonate which in turn acts 

 on the calcium sulfate of the sea water. 



