1856,] SCROFE CRATERS AND LAVAS. 347 



melting and cooling of the clay-slates in place, in the manner sug- 

 gested by Sir C. Lyell. 



In the formation of the clay-slates, perhaps, the action of heat was 

 not concerned (except as engendering the pressure to which they 

 have evidently been subjected), but that of water or an aqueous 

 silicate only. Still in their case also internal movements and mutual 

 friction of the component particles under extreme and irregular 

 opposing pressures have, I am convinced, had a primary influence in 

 occasioning that parallel arrangement of the scaly and flaky mica- 

 ceous particles to which their slaty cleavage is due. This, at least, 

 was the conviction forced upon my mind by a close examination of 

 the fissile clinkstone of the Mont Dor and Mezen, which is used for 

 roofing-slate, and is in its lamination and cleavage undistinguishable 

 from many clay-slates. And that opinion I recorded at the time in 

 my * Considerations on Volcanos *.' 



I have since found this view of the origin of slaty cleavage sup- 

 ported by Mr. Darwin in his work on * Volcanic Islands,' and by 

 Mr. Sorby in his paper on slaty cleavage in the Edinburgh Philoso- 

 phical Journal for 1853. I need not say that such support afl'ords 

 strong confirmation of its correctness. 



Of course we are led to connect the movements under extreme 

 pressure, to which this peculiar texture of the laminated rocks is here 

 attributed, with the action of those same forces by which their beds 

 have been so generally bent and contorted into a series of folds or 

 wrinkles, more or less at right angles to the general strike. 



If we seek to discover under what circumstances these flexures 

 were brought about, we can hardly be wrong in ascribing them to 

 the same violent process by which they have been elevated, usually 

 on the flanks of some protruded ridge or enormous dyke of crystal- 

 line rock, which is seen to form the axis of the mountain-range to 

 which they belong. 



Now what may we suppose to have been the character of this ele- 

 vatory process ? 



The phaenomena of active volcanos, and the protrusion of intumes- 

 cent crystalline matter on so many points of the earth's surface, and 

 at all periods of its history, may be admitted to prove the continued 

 existence beneath a very large area of that surface — if not the whole 

 — of a mass of intensely heated crystalline matter, having dissemi- 

 nated throughout its substance (in the manner already dwelt upon) 

 some fluid or fluids, such as water, affording an imperfect liquidity 

 to the mass, and, by its intense elastic force, communicating to it a 

 powerful tendency to expansion. Now suppose any considerable di- 

 minution to occur locally in the amount of pressure confining this 

 expansible mass beneath the crust of the globe, — such as might be 

 brought about by any extraordinary concurrence of the ordinary 

 barometric, tidal, oceanic, or excavating causes (not to suggest 

 others), — or, on the other hand, any considerable increase of its ex- 

 pansive tendency, owing to a local increase of temperature, from some 



* See pp. 103, 144, and 202. 

 VOL. XII. PART I. 2 B 



