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, | have traced the lime- 
rock that crosses the Susquehannah, at Harrisburg, 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. 
Thave 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.+ 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. 
