Geological Society. 315 
parallel to such planes. The planes of stratification, on the other 
hand, are perfectly distinct from both, and throughout the district 
alluded to have never been found to coincide with the lines of cleav- 
age, dipping sometimes to the same point and sometimes to opposite 
points of the compass, but being always inclined to them at an angle 
of from 10° to 30° or 40°, and inno instance at 90°. There are re- 
gions in North and South Wales thirty miles in extent, and many 
miles in breadth, where the cleavage planes preserve an undeviating 
dip and direction notwithstanding that they traverse strata which 
are greatly contorted. — 
In that variety of slate-rock which is used for roofing, all traces 
of original deposition or stratification are often obliterated ; yet in 
many quarries, a number of parallel stripes are discovered, sometimes 
of a lighter and sometimes of a darker colour than the general mass. 
These stripes, says the Professor, are universally parallel to the 
true beds, whenever such beds can be discovered, whether by or- 
ganic remains, by the alternations of similar deposits, or other 
ordinary means. Many of these beds are of a coarse mechanical 
structure, others are fine chloritic slate; but the coarser beds and 
the finer, the twisted and the straight, have all been subjected to 
one change, a crystalline cleavage passing alike through all. Some 
of the sections given show the cleavage planes preserving an almost 
geometrical parallelism while they pass through curved strata, of 
which the sedimentary origin is obvious. In another place it 
is said that where the slaty cleavage is very perfectly brought 
out the rocks always make an approach to homogeneity, but where 
the coarse beds predominate the slaty structure almost entirely dis- 
appears. Dr. Boase in his comments on these passages has re- 
marked that they seem inconsistent with each other, and I confess 
that at first they struck me in the same light ; but the Professor has 
explained to me that although the coarse beds are not slaty, they 
have a grain parallel to the cleavage planes of the finer beds, this 
grain being exhibited when they are struck with the hammer; and it 
is only when the materials of the beds are very coarse that the 
cleavage planes entirely vanish. : 
In regard to the origin of these phenomena, the author supposes 
that crystalline or polar forces must have acted on the whole mass 
simultaneously in given directions, and that the action being carried 
on at once through a very large mass of matter may have acquired 
an accumulated intensity of crystalline action in each part, so that 
the whole intensity of crystalline force, modifying the mass, may 
not have been equal to the sum of the forces necessary to crys 
stallize each part independently, but may have been some function 
of that sum whereby it may have been increased almost indefinitely. 
I regret that I have not space to do justice to this ingenious 
speculation, nor have I yet had sufficient opportunities of obser- 
vation to know whether we shall be able to distinguish generally, 
with precision, those slates which are diagonal to the strata, from 
those flagstone-slates, as it is proposed to term them, which are pa- 
rallel to the layers of deposition. Pe the lastsummer I observed 
2K 2 
