568 Wm. F. Prouty — Meso-Silurian Deposits of Maryland. 



diatelj above the top Clinton iron-ore sandstone contain a 

 greater percentage of Rochester fossils than of those of the 

 pre-Rochester Meso-Silurian, and consequently it would appear 

 that the lower limit of the Rochester should be drawn at the 

 top of the heavy sandstone layer. The thickness of the Clin- 

 ton does not vary far from five hundred and fifty feet. 



The Later Meso-Silurian (Rochester). 



The rocks of the Later Meso-Silurian formation, as herein 

 discussed, include those lying between the heavy band of sand- 

 stone bearing the top Clinton iron-ore and the bed of disinte- 

 grated yellow rock full of Zeperditia which marks the base of 

 the overlying Cayugan, and are composed for the most part 

 of thin-bedded limestones with shale partings. For a short 

 distance above the bottom and for a greater thickness near the 

 top the shales increase and preponderate over the limestone. 

 The lowest limestone layers are grayish blue in color and, 

 together with the shales which immediately, overlie the Clinton 

 sandstone layer, are very fossiliferous. The upper limestones 

 are uniformly of a darker color and more compact, sometimes 

 occurring in more or less lenticular beds with thin shale part- 

 ings. In general the middle limestones and shales contain few 

 fossils, with the exception of Ostracods and a few Favosites 

 and Orthoceras. Toward the top, however, the brachiopod 

 life begins again to abound and some beds are very fossilif- 

 erous. The upper shales are usually darker hi color than those 

 below and often become arenaceous, bearing thin sandstone 

 lenses, while the top of the formation is often formed by a bed 

 of sandstone of variable thickness which is often very ferru- 

 ginous. These upper dark shales usually carry a great number 

 of Ostracods and poorly preserved Bryozoa. 



Going toward the east, one generally finds that both the 

 lower shales and the upper ferruginous sandstone layers 

 iu crease in thickness. The thickness of the formation is in 

 general not far from three hundred feet. At Pinto, where the 

 most accurate measurements were made, though they are not 

 entirely satisfactory because of some faulting, these rocks show 

 a thickness of two hundred and eighty-eight feet. 



General Lithological Relations. 



It is concluded from a comparative study of the different 

 exposures in the state that during the deposition of these 

 sediments there existed a shore line not far to the east, and a 

 gradual deepening of the waters toward the west as far as the 

 section exposed at Pinto (see section, fig. 2 and map, fig. 1). The 



