368 
THE GEOLOGIST. 
and irregular pressures to which, during their elevation, they must 
have been subjected. When the component particles of any bed were 
of an argillaceous character, and therefore capable of combining with 
water into a slippery paste, or where a large proportion of them were 
of a flattened form, as in the micaceous shales, they will necessarily 
have yielded so readily to oblique pressures, by sliding or slipping 
over each other, as to give rise to a highly lamellar arrangement, the 
planes of the laminas taking the direction of the movement. Moreover, 
the entire mass itself will have been proportionately extended, or 
flattened out, in the same direction, and contracted in its opposite 
dimension. And thus was probably occasioned the " cleavage " of the 
slate rocks. The artificial production of cleavage in wax by Professor 
Houghton, and of laminated stone by Mr. Sorby, have shown experi- 
mentally that pressure will give rise to this lamellar structure in 
substances composed of scaly or flaky particles possessing a certain 
mobility inter se. 
But while the micaceous or argillaceous beds were thus yielding to 
pressure, and stretching their dimensions in a direction perpendicular 
or diagonal to it, the coarser conglomerates and calcareous strata, whose 
component solid particles did not allow of so much internal motion, 
will have been rather broken up by the formation of cracks and crevices 
throughout their substance, under the influence of irregular pressures. 
And in this manner, probabl)^, quite as much as by shrinkage, was pro- 
duced the veined and brecciated structure of such of tliese rocks as 
have been subjected to considerable disturbance — viz., the veined 
marbles, some serpentines, the calcareous breccias, and quartzose sand- 
stones. 
In every class of rock, I maj' observe, whether igneous or aqueous, 
observation will, I think, show tliat the internal structure which I am 
liere referring to internal friction accompanying movement under 
extreme pressure, preceded the concretionary separation of the 
substance by joints or scams into prismatic or other divisional forms. 
I will not pursue the subject further at present, my object being 
merely to urge the attention of geologists, especially when working in 
the field, to the mechanical influence that must have been exercised on 
the internal structure of rocks of all kinds which have sufi'ered much 
change of position since their formation, by the internal movements 
occasioned in them by intense and irregular pressures. 
