DETAILS OF THE SECTIONS 491 



There are thus 20 feet of strata between the top of the Niagara and the 

 black shales carrying the Euryptericls. The stromatoporoid reef rocks, 

 with an incipient Gruelph fauna, were succeeded by thin-layered, impure, 

 ripple-marked dolomites, with "fucoids" as the only common fossil. This 

 was followed by the deposition of tough porous, very bituminous lime- 

 stone, succeeding to which was formed a stratum of thin-layered bitumi- 

 nous accretionary limestone, forming flat, imbricating, shell-like domes. 

 Then another change resulted in the formation of 2 feet of light yellow- 

 ish, impure limestone, with indications of a finely branching organism, 

 probably a plant, and followed by a greenish, shalv marlite, which in 

 Sarle's view form the base of thc'Salina, and carries a small species of 

 Pterinea. The water-lime next succeeding, with its shrinkage cracks, 

 marks a temporary drying up of the region, though marine conditions 

 are still indicated by the occurrence of a Pterinea, a Lingula, a Leper- 

 ditia, and an Orthoceras. With the exception of the last, these fossils 

 have been found in the s black shale layer which is chiefly characterize!! by 

 the Eurypterid remains. Above this horizon the green shales and dolo- 

 mites continued for a while and then gave way to red sedimentation. 



Among the species found in the black shale IlughniiUeria socialis 

 Sarle, a species very closely similar to II. shawangunk Clarke of the 

 Eurypterid shales of the Shawangunk conglomerate, takes the first rank. 

 It occurs abundantly in the lower half of the black shale, "where it is so 

 prolific that certain planes are literally packed with its remains, making 

 this probably the richest Eurypterid stratum known." Another form, 

 variety roousta Sarle, also occurs. The genus Eurypterus, represented 

 by the single species E. pittsfordense Sarle, occurs most frequently in the 

 upper half of the shale, just above the limestone parting. The dolomite 

 layers, on the other hand, are almost entirely free from Eurypterid re- 

 mains, but contain Leperditia, Pterinea, and occasional pentalabiate 

 Gomphoceras. Other species found are Pierygotus monroensis Sarle. 

 Stylonurus mullispinosus Clarke and Ruedemann, Ceratiocaris prcecedens 

 Clarke, Emmelozoe decora Clarke, and Pseudoniscus roosevelti Clarke, 

 besides fragments of other species. 



The significance of this section will be discussed later, hut there arc 

 several points to which attention may he called now. It is evident that 

 some marked changes was taking place with the beginning o\' Pittsford 

 sedimentation. The old fauna, which had existed over this region for a 

 long period of time, was dying out. Temporarily a Dew assemblage o( 

 organisms appeared, whose remains were, moreover, restricted to (he 

 black muds, while the dying Niagaran fauna made several attempts to 



