GEOLOGIC FORMATIONS OF NEW YORK 161 



stones seen in the northern counties and generally around the 



edges of the different coal fields. In southern Pennsylvania it 



includes limestones. This formation has not been recognized in 



New York. 



Pottsville conglomerate 



The Mauch Chunk red shale is covered by a thick series of 

 strata, known as the Pottsville conglomerate. It is a gray and 

 whitish conglomerate, in massive beds alternating with gray 

 sandstones, and consists mainly of rolled and rounded quartz 

 pebbles cemented with ferruginous sand into a solid mass. Some 

 of its finer or more sandy layers often show lamination in a 

 diagonal or slanting direction. It is 1,700 feet thick at its maxi- 

 mum and often contains one or more thin seams of coal; being 

 the lowest horizon in which any considerable quantity of that 

 mineral has yet been found. It is remarkably massive in its 

 general appearance, the ledges often separating into huge blocks 

 with wide fissures between, which have been fancifully compared, 

 to ruined cities. Such localities are to be seen in New York six 

 miles south of Olean, seven miles south of Ellicottville and near 

 Wellsville, where they are popularly called ' rock-cities.' This 

 is locally known as the Olean conglomerate. 



The ' rock cities ' lie on high points not far from the Pennsyl- 

 vania line and are simply remnants of the conglomerate left far 

 north of the main body of the rock by the wear and tear of the 

 elements, which, going on through ages, has worn away this mas- 

 sive stratum over a great extent of country. They are impressive 

 monuments to the vastness of that erosion, which has left them in 

 this isolated position and which will in the course of future 

 centuries demolish them entirely. 



This conglomerate is the highest and latest formed of all Palae- 

 ozoic rocks known within the limits of New York. In Pennsyl- 

 vania it is the base of the 'Productive Coal-measures,' as 

 the strata containing workable layers of coal are called. 

 They are made up of thick beds of sandstones and black 

 shale, with which the coal layers are interstratified. The coal 

 strata are of all thicknesses, from a few inches up to 20 or even 



