■ 



226 Geology of the Country between 



and suddenly reversed, although the general tendency still continues 

 to be towards the S. E. In the arenaceous varieties the planes of 

 stratification are very distinct, and cannot well be misunderstood. 

 In the slaty varieties, there would seem to be a little more difficulty. 

 We are told by Bakewell, that " slate invariably splits in a transverse 

 direction to that of the beds, making with that direction, an angle of 

 about 60°"; he excepts however from this rule, " coarse gray wacke 

 slate and soft slate or shale." Now the greater part of the slate of 

 the region of which I am speaking, is so soft that it might properly be 

 called shale ; whether this is sufficient to account for the fact, or 

 whether the above transverse cleavage really exists and has been 

 overlooked, the simple fact is, that nothing of the kind has been no- 

 ticed. The rock in this respect agreeing with the proper argillaceous 

 slate of Massachusetts, as described by Prof. Hitchcock in his late 

 report. He observes* "excepting in the argillaceous slate connected 

 with the graywacke, I have not been able to find in this rock planes 

 of stratification, moving in a different direction from the laminae, a 

 circumstance very common it is said in Europe. But in general, 

 strata seams are discoverable, lying parallel to the slaty structure, as 

 in mica slate. The slate indeed contains numerous seams not coin- 

 cident with those of the strata, but there is rarely any continuous par- 

 allelism among them." Even in regard to the " argillaceous slate 

 connected with the graywacke," that is excepted from the above re- 

 marks, it would appear that this peculiar structure is far from being 

 uniform, indeed I should rather infer it was only an occasional occur- 

 rence, and looked upon when occurring, as an exception to a differ- 

 ent rule. "The slaty structure of the slates, included under gray- 

 wacke, does not always coincide with the stratified structure."! The 

 next stratum west is the limestone of Frederick valley, and this ex- 

 hibits a dip and direction similar to that of the graywacke, the two 

 are not seen in actual contact along the line of the section, but from 

 their close approach, the observer can have but little doubt that the 

 former actually passes under the latter. This opinion was stoutly 

 contested for a long time, as being opposed to analogy, but account- 

 able or unaccountable, such is the fact. The most convenient point 

 to inspect the near approach of the limestone and slate, is near the 

 viaduct of the Baltimore and Ohio rail road across the Monocacy ; 

 on the left bank is a high bluff of slate and on the right, a little farther 

 up the stream, are the bassetting edges of true sparry limerock, the 



Report on Geology, &c. of Massachusetts, pp. 289, 290. t lb. 273. 



