Baltimore and the Ohio River. , 229 



graywacke slate, and coarse gritstone." Some of these beds are of 

 considerable thickness, and form mountain masses. De La Beche, 

 also speaks of patches of limestone, often continuous for considera- 

 ble distances, intermingled with the arenaceous and slaty rocks of 

 the graywacke series. Again he speaks of the limestones having a 

 series of sandstones and slates, similar for the most part to those be- 

 neath, accumulated above. "In some districts, such as the north of 

 Devon, there has been a return of causes, favorable to the deposit of 

 limestone, and two bands parallel to each other have been produced. 

 In other districts, more limestones have been formed, while in some 

 they are nearly absent ; a state of things we should expect from va- 

 riations produced by local circumstances, or similar general causes 

 in operation over a considerable area."* These remarks may be 

 very appositely applied to the transition district on our section. We 

 there see a very extensive graywacke deposit, often interrupted by 

 patches of limestone, occurring very irregularly and of very variable 

 extent. Until we are able to ascertain what those general causes 

 were that operated to produce such calcareous deposits, we must be 

 content to remain ignorant of the reason of this irregularity. The 

 term bed, appears too trivial to be applied to deposits so extensive, 

 {longitudinally,) as our limerock, since the term is most usually be- 

 stowed upon such as are very circumscribed. It seems preferable 

 to consider the rock in question, as interstratified with the graywacke 

 strata. As only one example of its extent, I have traced the lime- 

 rock that crosses the Susquehannah, at Hairisburg, Pa., southwester- 

 ly and southerly in the direction of the valley, through Carlisle and 

 Chambersburgh,Pa., Hagerstown, Md., Martinsburg and Winchester, 

 to Woodstock, Va. With the exception that I did not pass in a di- 

 rect line from Hagerstown to Martinsburg, the identity of the rock as 

 seen at those two places, is inferred from its perfect agreement in 

 the direction and inclination of the strata and in its external characters. 

 I have no information how far this stratum extends N. E. of Harris- 

 burg, Pa., probably to the Delaware River or beyond, perhaps it 

 may even be connected with the blue cherty limerock of Orange Co. 

 N. Y., described by Mr. Charles U. Shepard.f He found that in- 

 timately associated with an argillite, described as very similar to 

 what exists in the same connexion in these parts and also connected 

 with a white crystalline limerock. What agency the gneiss and sie- 

 nite of Orange Co. might have had in converting the limestone to 



* Geological Manual, p. 437. t See Amer. Jour, of Science, Vol. xxi, p. 321. 



