RocTcs and Fossils of PhilUpsburgh, C. E. 311 



Although I have searched a good deal for fossils I have not found 

 any either in the slates or magnesian limestones. 



2. Blue thin-bedded and Nodular Limestones. 



Lying to the east of the magnesian limestones, and above them, 

 is a formation of greyish or dark blueish, sometimes almost black 

 limestone, with some beds of white marble, of limited extent at 

 the base. The darker coloured limestones, which constitute nearly 

 the whole of the mass, consist of beds of from three inches to three 

 or four feet in thickness. Usually the thicker beds seem to be 

 composed of a number of thin layers, with irregular thin seams of 

 shale between. Many of them thus present a nodular appearance. 

 In this deposit there are occasional magnesian beds interstratified. 

 Some of the strata are silicious, and where exposed to the action 

 of the atmosphere, lose their lime, the residue forming a light red 

 friable maSjS, in which the forms of the fossils are well preserved, 

 either as casts of the interior or exterior. The thickness of these 

 limestones has not yet been ascertained, but it is probably not less 

 than 400 feet. The strata are a good deal disturbed by faults, 

 and much further examination will be required before it can be 

 determined with certainty, how often the same strata are repeated 

 in the different ridges. On a recent visit to this locality with Sir 

 W. E. Logan, we found in these rocks about forty species of fos- 

 sils, which shew that this part of this series of limestones is the 

 equivalent of the upper part of the Calciferous sandrock. Of 

 these fossils I shall now proceed to give an account. 



Plant^e. — Several species of fucoids occur on the surfaces of 

 some of the strata. They resemble those of the Calciferous sand- 

 rock. 



ZooPHYTA. — One specimen was found which resembles Steno- 



para fibrosa^ and in the higher beds, an obscure fossil very like 



Tetradium fihratum. These fossils are so badly preserved that I 



do not consider them identified. No trace of any other coral was 



observed. 



Echinodermata. — There are here the columns of three or 

 four species of Crinoids. The detached plates of a Cystidean 

 which is either Paloeocystltes tenuiradiatus, (Hall, sp.) so abun- 

 dant in the Chazy limestone, or a closely allied species — is com- 

 mon. 



Bryozoa. — In the highest beds several specimens were ob- 

 served which resemble Stromatopora rugosa, (Hall, sp.) but it 



