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and argillaceous impurities. Estimating the formation from the drill 

 cores that have penetrated it, and placing the lower limit as nearly 

 as may be at the horizon of changes from predominant lime to pre- 

 dominant silicious content, the approximate thickness in this region 

 is placed at 200 feet. The rock where exposed exhibits considera- 

 ble joint development and these are considerably enlarged by the 

 solvent action of percolating waters. This factor is considered of 

 some importance in connection with the other limestones of the 

 district in aqueduct construction and permanence. The Onondaga 

 has been used as a building stone formerly sold as marble, some 

 grades of which are good stone. On the line of the aqueduct it is 

 confined to the Rondout and Esopus valleys. The chief fossils are : 

 Atrypa reticularis, Zaphrentis prolifica, 

 Leptostrophia perplana, Platyceras dumosum, 

 Leptaena rhomboidalis, Dalmanites selenurus. 



(11) Esopus and Schoharie shales (a slaty grit). The Schoharie 

 as a distinct formation is not distinguishable in this region. The 

 very thick and comparatively uniform, gritty, black, dense, almost 

 structureless rock is a distinct unit. It is a silicious mud rock with 

 very obscure sedimentation markings, but showing independent 

 secondary cleavages induced by later dynamic factors, and, on long 

 exposed surfaces always exhibiting chiplike fragments as the result 

 of weathering. But it is not an easily destroyed rock. In so far 

 as the bedding is obscure and the induced structure predominates, 

 the rock is a slate; and in so far as it is distinctly gritty (sandy) 

 instead of argillaceous it is a grit. The formation might therefore 

 be more accurately designated as a slaty grit. The lack of plain 

 bedding structure makes it impossible to estimate its thickness, 

 since the foldings or other displacements can not be allowed for ; 

 but the accumulated data of drill holes in more advantageous 

 position indicate an approximate thickness of 800 feet. The rock 

 is considered exceptionally good ground for the tunnel. 



A few fossils occur the most characteristic being Taonurus 

 c a u d a g a 1 1 i . There are also in certain layers of limited extent, 

 Leptocoelia acutiplicata and Atrypa spinosa. 



(12) Oriskany and Port Ewen transition (silicious shaly lime- 

 stone). There is no well defined and distinct separation here be- 

 tween the Oriskany and the underlying Port Ewen, but because of 

 the importance and persistence of the formation in other and re- 

 lated areas the name is held. The equivalent of the Oriskany is in 

 this district involved with a strongly developed transition zone 

 which in physical features is intimately associated with the Port 



