50 J. J. STEVENSON — LOWER CARBONIFEROUS, APPALACHIAN BASIN 



The same conditions prevail in the Conemaugh gap through the Laurel 

 Hill axis, where the whole of the limestone is brought up and seen to be 

 much thicker. In the Conemaugh gap through Chestnut hill, within 

 Westmoreland county, J. J. Stevenson found along the Pennsylvania 

 railroad this succession : 



Feet 



1. Shales 82 



2. Fossiliferous limestone i> 



3. Shales and sandstone 60 



4. Calcareous shale 3 



5. Interval 37 



6. Silicious limestone 40 to 50 



The same section is shown on the Indiana side of the river.^ In that 

 county limestone is present in Chestnut hill almost midway from the 

 Conemaugh to the Clearfield line. In this region one finds for the first 

 time along the lines thus far followed full exposures of this limestone, as 

 farther north and east the deposit is much thinner. It may be well to 

 introduce here a detailed description of it as given originally by Stevenson : 



"The upper portion is a conglomerate sandstone of varying thickness, in which 

 are great numbers of more or less angular fragments of the silicious limestone. This 

 passes down imperceptibly into the silicious limestone proper, which in its turn 

 passes into the Pocono sandstone below. The fragments in the conglomerate por- 

 tion are so clearly free in most cases from all traces of aqueous action that they 

 may have been stripped off by some abrading agent while the limestone bed was 

 above water level. 



** The limestone itself is an exceedingly fine grained rock, with a delicate blue 

 color, and on the fresh surface it shows no lines of bedding. It has a flint like 

 fracture and no definite cleavage. On the long exposed surface the color is dull 

 brown and the rock resembles a loose sandstone. Under such circumstances the 

 structure of the rock is perfectly distinct, and it shows curious cross-bedding. 



" At first glance this rock is hardly to be taken for a limestone, and the silica 

 evidently predominates at all localities. At the same time lime is present in con- 

 siderable quantity, for when the rock is burned it becomes snow white, slakes 

 readily, and forms a mortar without the addition of sand."t 



The upper fossiliferous limestone is shown in the Conemaugh gap 

 through Chestnut hill, and its thickness suggests that the beds ought to 

 be found at some distance farther north, but Mr Piatt appears to have 

 found no traces of it in Black Lick gap of Indiana county. It was not 

 observed in the gap through Laurel hill. The section recalls that ob- 

 tained by Doctor White in Broad Top, for here one has the upper shales 



*J. J, Stevenson : Report of progress in the Fayette and Westmoreland district, part ii (K 3), 

 1878, p. 48. 



t J. J. Stevenson, vol. i (K :j), p. 52. The statement on page 53 respecting the distribution of 

 this deposit is wholly erroneous. 



