ROCHESTER AND ONTARIO BEACH QUADRANGLES 25 



In the Tanner quarry Poleumita scamnata has been 

 found and several specimens of this species were obtained from the 

 Trabold quarry, a few miles west of Rochester. It appears quite 

 probable that we have other earlier manifestations of the Guelph 

 fauna in the vicinity of Rochester and that patient search may 

 reveal the presence of additional species that would bear some 

 relation to the lower Shelby fauna. 



Scattered over the surface of the country about Rochester, are a 

 number of Lockport boulders which weather brown, and contain 

 cavities which give the rock a very scraggy appearance. Occasion- 

 ally on some of these boulders fossils are found standing out in 

 relief. On breaking the rock, the glistening surface and petroleum 

 odor are very noticeable. 



The physical conditions during late Lockport time are of special 

 interest for they involve the last of the true marine deposits of 

 Siluric time, and throw much light on the presence of the Guelph 

 fauna in the dolomites, as well as accounting for the high magnesian 

 content of the rock. We must conceive of a shallowing Lockport 

 sea, gradually becoming more inclosed. This was accompanied 

 by an increase in the saline and magnesian content of the sea 

 water. Under these conditions coral reefs became very abundant 

 and favorable for the existence of life under new conditions and 

 environments. Clarke and Ruedemann 1 state that the chert-bearing 

 dolomite is highly magnesian, containing about 44% of magnesian 

 carbonate. 



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

 gives off a strong petroleum odor when fresh or when 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. We may note that the admixture of bituminous mat- 

 ter in these Guelph dolomites is a further indication of their coral 

 reef origin, or is at least in harmony with recent observations on 

 living coral reefs where petroleum has been found in process of 

 formation as a result of the transformation of the organic matter of 

 the reef. The cavernous character of the dolomite may, according 

 to the views of Walther and others, be regarded not as the result of 

 subsequent corrosion, but as the remnants of original cavities in the 

 growing reef which have not been closed up with coral sand. 



The chert concretions which are characteristic of the upper 

 Guelph horizon at Rochester and Shelby are doubtless a by-product 

 of the diagenesis which altered the coral lime rock to a dolomite. 



iN, Y, State Mus, Mem. 5. 1903. p. 115, 116. 



