G. P, Scrope on Craters, and the Liquidity of Lavas. 227 



I have since found this view of the origin of slatj cleavage 

 supported by Mr, Darwin in his work on ' Volcanic Islands/ and 

 by Mr, Sorby in his paper on slaty cleavage in the Edinburgh 

 Philosophical Journal for 1853. I need not say that such sup- 

 port affords strong confirmation of its correctness. 



Of course we are led to connect the movements under extreme 

 ressure, to which this peculiar texture of the laminated rocks is 

 ere attributed^ with the action of those same forces by which 

 their beds have been so generally bent and contorted into a se- 

 ries of folds or wrinkles, more or less at right angles to the gen- 

 eral strike. 



If we seek to discover under what circumstances these flex- 

 ures were brought about, we can hardly be wrong in ascribing 

 them to the same violent process by which they have been ele- 

 vated, usually on the flanks of some protruded ridge or enor- 

 mous dyke of crystalline 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 

 elevatory process ? 



The phenomena of active volcanos, and the protrusion of in- 

 tumescent 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 crys- 

 talline matter, having disseminated throughout its substance (in 



the manner already dwelt iipon) 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 diminution to occur 

 locally in the amount of pressui'e confining this expansible mass 

 ' " ■ - . . . g^^^j^ ^^g naigbt be brought about 



beneath . .„., ^ , ^ ^ . 



ty au extraordinary concurrence of the ordinary barometric, 

 tidal, oceanic, or excavating causes, (not to suggest others),— or, 

 on the other hand, any considerable increase o^ its expansive 

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

 'inknown, but easily imagined, cause,— we should anticipate, 'as 

 the necessary result, the violent fracture and elevation of the 

 overlying crust of rocks, and the extrusion through some prin- 

 cipal fissure, or line of fracture, of a ridge of the subterranean 

 intumescent crystalline matter. 



It seems very probable tliat under such circumstances the cen- 

 tral axis of the protruded ridge may retain its irregularly crys- 

 talline grain and structure, but that the portions of crystalhne 

 flatter that from either side would rush or be thrust up by pres- 

 sure from behind (consisting partly of the weight of the overlying 

 rocks on the semi-liquid matter below them) towards the opening 

 should be subjected to so much internal friction of their cry sUl- 



f 



.; 



