3(58 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 laminae 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 rochs. 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 i7iter se. 



Eut 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, probablj-, quite as much as by shrinkage, was pro- 

 duced the veined and brecciated structure of such of these rocks as 

 have been subjected to considerable disturbance — viz., the veined 

 marbles, some serpentines, tlie . calcareous breccias, and quartzose sand- 

 stones. 



In every class of rock, I may observe, whether igneous or aqueous, 

 observation will, I think, show that the internal structure which I am 

 here referring to internal friction accompanying movement under 

 extreme pressure, preceded the concretionary separation of the 

 substance by joints or seams 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 sufl'ered much 

 clmngo of position since their formation, by the internal m.ovements 

 occasioned in them by intciise and irregular pressures. 



