340 Correspondence — Mr» G. Poulett Scrope. 



either end, and broken by upward pressure at its centre, namely, a 

 compression which takes effect in the central part beneath, and in the 

 lateral parts adjoining the fixed extremities above a neutral line or 

 * pivot axis ;' while, on the other hand, the upper portion of the 

 central parts and the lower strata of the lateral parts will be sub- 

 jected to a tearing strain." The resulting effect of such dislocating 

 forces on rocks of so rigid and coherent a character as to break rather 

 tlian bend, or yield like a liquid or pasty mass under pressure, would 

 be to cause rents at right angles to the direction of the dislocating 

 force and "gaping," i.e. widening upwards, in the case of an eleva- 

 tory action about the centre and downwards about the lateral portions, 

 and vice versa, of course, in the case of a depressing action. If any of 

 these rents opened so far downwards as to reach a mass of matter 

 liquefied by heat, or at such a temperature as to be more or less lique- 

 fied by the reduction of pressure to which under such circumstances 

 it would be exposed, the result would be the suction or pumping up of 

 such liquefied or pasty matter into the fissures, and should any of 

 these reach the outer atmosphere, its explosive eruption on those 

 points. While, on the other hand, in the portions subjected to com- 

 pression the result would be the contortion and outward bulging 

 of such masses of rock as were pliable, and the dislocation and 

 outward shoving of wedge-shaped portions of such rocks as were 

 too rigid to yield otherwise, much "as we see wedge-shaped chips 

 split off and forced outwards from the edges of a crack formed 

 in the same relative positicm through a rigid mass of stone or 

 metal broken across by pressure" (p. 54, op. cit). 



I will not occupy more space in your pages by an extension of 

 these quotations. But your readers will, I think, admit that these 

 considerations may serve to throw some light on the questions re- 

 lating to the probable origin of the fissures, faults, veins, dykes, con- 

 tortions, and other obvious displacements of superficial rocks, to 

 which Mr. Maw's and Mr. Wilson's observations refer. In the work 

 above cited I have ventured to carry still further the speculations 

 they suggest, by hazarding the supposition that, when repeated, 

 elevatory movements have operated through a long time over very 

 wide areas, the result may be seen along the central axes of disloca- 

 tion in some of the chief mountain ranges of our continents, while 

 distant parallel lines of volcanic development mark the horizontal 

 limits of disturbance on one or both sides, where the production of 

 rents gaping downwards may have allowed the heated subterranean 

 matter to force its way up towards, or actually to the subaerial sur- 

 face. These last speculations must go for what they are worth. 

 But the mechanical theory on which they rest can hardly, I think, 

 be disputed. It differs in some respects (as I have shown in 

 Volcanos, p. 51) from those of Mr. Hopkins and Mr. Darwin, but 

 rather in the terms of the question, that is, the supposed circum- 

 stances, than in its solution. With regard to outward bulging or 

 contortion, accompanied as it must be by movement and friction, 

 inter se of the particles, should they be capable of movement, 

 being the cause of slaty cleavage, I have always agreed with 



