34 NEW YORK STATE MUSEUM 



beds northward is due to a thinning out of the exposed portion of 

 the strata, as can be readily seen in the Lockport limestone bed, 

 which is less than 30 feet thick at Lewiston, but more than 80 feet 

 at the falls, increasing in thickness southward to 250 feet or more. 

 Where, however, the strata are not exposed on the surface, i. e. 

 where they are only shown in sections under cover of the overlying 

 rock, no such thinning is seen. This may be observed in the case 

 of the Clinton beds and the upper Medina sandstones. In some 

 cases these beds are seen to even thin southward, as proved by bor- 

 ings. The thinning of these strata does not, as is often assumed, 

 mark the original thinning of the beds toward the shore on the 

 north, but is evidently due to erosion. A brief resume of the origin 

 of the various strata will make this clear. 



The Medina sandstone is an ancient shore and shallow water de- 

 posit, as will be more fully pointed out in chapter 3. The 

 sands and gravels, which with some finer muds, make up this 

 rock, are all derived from some preexisting land. The only 

 source of supply was the old Laurentian land on the north 

 and the Appalachian old-land on the south. It is true that, 

 owing to the elevation at the beginning of Siluric time, some 

 of the pre-Siluric stratified rocks may have been raised above the 

 sealevel and added to the old-land, and that part of the Medina 

 sands may have been derived from these. Even then the largest 

 amount of detritus was probably derived from the crystalline old- 

 lands, the progressive accumulation of 1200 feet of Medina rock 

 marking a corresponding subsidence and a concomitant encroach- 

 ment of the seashore of the Medina sea on the old-land. Thus the 

 Medina deposits gradually overlapped the Ordovicic and Cam- 

 bric deposits and probably eventually came to rest entirely on the 

 crystalline pre-Cambric rocks. Continued subsidence, at least in 

 the Niagara region, produced the purification of the water, so that 

 eventually the limestones of the Clinton epoch could be formed in a 

 region remote from that in which terrigenous material was ac- 

 cumulating. This was likewise true of the Lockport limestone, 

 which was deposited after an interval, during which the calcareous 

 shales separating the two limestone series accumulated. While 



