LOWER SILURIAN SYSTEM OF EASTERN MONTGOMERY CO. 465 



With the Black river limestone the case is more difficult. In 

 truth it is only under favorable circumstances that the Black river 

 can be clearly distinguished from the overlying Trenton. In sec- 

 tion 46A the Columnaria zone has a thickness of from 7 

 to 9 feet and is followed by a conspicuously darker-weathering, 

 semi-crystalline limestone with an appreciable stratigraphic break. 

 In section 2B the contrast between this zone and the basal member 

 of the Trenton substage is even more marked, since in the latter 

 section the Trenton is looser grained and weathers darker than in 

 the western part of the region. In section 2D there is a decided 

 contrast in color between the Columnaria zone and the sub- 

 jacent Birdseye, but no stratigraphic break. The two appear to 

 constitute one layer, a fact which is also very noticeable at Newport, 

 N. Y. in the quarries about two miles north of the village. The 

 thickness of the Black river in 2D is apparently only 3^ feet, 

 and it will be noticed that in this part of the region the massive 

 member of the Trenton substage is unusually thick. The lumpy 

 structure mentioned by the early New York geologists as a charac- 

 ter of the Columnaria horizon at Watertown is its most con- 

 stant lithologic character in the present region. The fauna of the 

 Black river limestone is of interest. The three fossils S t r o - 

 matocerium rugosum, Streptelasma profundum 

 and Columnaria alveolata never pass above it. The 

 brachiopoda so abundant in the immediately overlying Trenton are 

 very poorly represented in the Black river. The most common 

 brachiopod of the latter rock, Zygospira recurvirostra, 

 is uncommon or lacking in the former. Rafinesquina al- 

 ter n a t a may usually be found in small numbers in any exposure 

 of Black river well adapted to collection. The obscure bryozoa of 

 the genus Stictopora are usually present. M o n t i c u 1 i - 

 pora (Prasopora) lycoperdon occurs associated with 

 the Co 1 u m n a r i a but is invariably the branched form, while in 

 the upper member of the Trenton substage where that fossil is 

 so abundant it is the hemispheric form that is common. These 

 faunal differences between the Trenton and Black river substages 

 are constant throughout the region in spite of variations in 

 lithologic characters. The affinities of the Black river limestone 

 arc^ however with the Trenton limestone and not with the 13ird§eye. 



