22S G, P. Scrope on Craters, and the Liquidity of Lavas. 



line particles, and so mucli pressure at right angles, or nearly so, 

 to the direction of the movement, as mnst stretch and draw them 

 out into parallel planes, — just as happened evidentlj to the 

 striped and ribboned trachytes in the protruded dykes of Ponza 

 and Palmarola, This friction and pressure would be extreme, 

 of course, along the lateral parts of the protriided mass, that is, 

 the selvages of the great dyke; which, if the original mass were 

 gi'anite, w^ould thus appear composed of an axis of granite, 

 passing on either side into gneiss (or squeezed granite) and far- 

 ther on into mica schist. 



But every irregularity, whether on the large or the small scale^ 

 obstructing more or less the even motion of the layers, must 

 create a waving or contortion in them, especially in the planes 

 of slippery mica-plates, such as is exemplified even in hand- 

 specimens of the Ponza trachytes, and also on the largest scale 

 in the same locality. And the extreme irregularities of motion, 

 occasioned on the upper layers of the in tumescent mass by the 

 pressure and resistance of the overlying beds, may be expected 

 to carry their wayings still further, and at the throat of the fis- 

 sure, where the squeeze and jam of the protruded matters must 

 be at its maximum, to occasion those enormous and repeated 

 zigzag foldings of the laminated beds, so frequently observed in 

 mica and chlorite-schists in such positions. 



Meantime another influence would be similarly affecting the 

 overlying stratified rocks above, or on the outer flanks of the 

 elevated axis, namely their own specific gravity, urging them to 

 slide or slip laterally when tilted up at (perhaps) a considerable 

 angle on either side. The more compact and indurated strata 

 would be partly fractured into cliffy masses, partly broken up 

 into breccias and conglomerates by this movement; but the soft- 

 er beds, especially those which were saturated with water (per- 

 haps even yet under the sea,) or wlu'ch contained interstratified 

 beds of silt, shale, or clay, permeated with water, would glide 

 laterally awa^ from the axis in extensive land-slips, and be 

 wrinkled up mto vast foldings under the intense pressure com- 

 pounded of their own weight, and that perhaps of portions of 

 the protruded matter thrust against them, — in a manner very 

 ■similar to the contortions produced in the more crystalline lami- | 



nated rocks by the violent squeeze which accompanied their pro- 

 trusion. It may even be difficult to draw a line between the 

 effects of these two replicating and fracturing forces. But, to- 

 gether, they seem to me sufficient to account for most of the phe- 

 nomena of the kind observable in mountain chains. 



These are the ideas on this subject which I endeavored to 

 develope, though very imperfectly I am aware, in the more tlie- 

 orotic portion of my work on volcanos, so ofY^en referred to, and 

 thej were illustrated by a rude ideal section of an elevated 



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