112 NEW YORK STATE MUSEUM 



tlie rock is a banded lime mudrock, rather massively bedded and 

 breaks with a conchoidal fracture. It is bluish gray when 

 fresh but weathers brownish. In the lower portion are frag- 

 ments and small heads of Favosites helderbergiae 

 var. precede ns [fig. 7], which have passed up from the 

 Cobleskill. Above the cement bed are 42 feet of strata, mostly 

 lime mudrocks, but with frequent layers of a more arenaceous 

 texture. Many of the beds are very shaly, partaking almost of 

 the character of paper shales and containing a considerable 

 amount of argillaceous material. On weathering they leave 

 much clay behind. These beds are considered worthless in the 

 manufacture of cement. They are succeeded by 43J feet of 

 Manilas, which is here quarried for Portland cement, containing 

 on the average from 93 to 94^^ of calcium carbonate. The beds 

 are mostly thin and irregularly bedded calcareous mudrocks, 

 the lowest portion being of the type often called ribbon lime- 

 stones. They are similar to the underlying beds but of a purer 

 composition. A few heavy bedded, somewhat more arenaceous 

 layers occur which are known by the name of " curly " layers.^ 

 The thinner bedded strata are often rich in Tentaculites 

 gyra can thus [fig. 25] and Spirifer vanuxemi [fig. 

 24] and Leperditia alta [fig. 24a] are also common in 

 some layers [pi. 8, 20]. 



The term Manlius was applied by Clarke and Bchuchert to the 

 uppermost formation of the Siluric, which has long been known 

 by the name of Tentaculite limestone, froim the abundance of the 

 small pteropod, Tentaculites gyracanthus, which 

 often covers the weathered surfaces of the slabs of this rock. It 

 is chiefly a rock composed of impalpable lime mud. Most of the 

 beds are very firm and when struck with a hammer emit a ringing 

 sound. There are usually alternating heavy strata, in which 

 fossils are rather rare, and thin Ibedded layers, in which the three 

 fossils Tentaculites gyr acanthus, Spirifer van- 

 uxemi and Leperditia alta are common, though gener- 



1 See section p. 259. 



