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 distinet, 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 graywacke ~ 
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 Hothiee 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 lamine, 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 5 
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 Ib. 278. 
