348 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 23, 



unknown, 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 principal fissure, or 

 line of fracture, of a ridge of the subterranean intumescent crystalline 

 matter. 



It seems very probable that under such circumstances the central 

 axis of the protruded ridge may retain its irregularly crystalline 

 grain and structure, but that the portions of crystalline matter that 

 from either side would rush or be thrust up by pressure 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 crystalline particles, and so much 

 pressure at right angles, or nearly so, to the direction of the move- 

 ment, as must stretch and draw them out into parallel planes, — 

 just as happened evidently 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 

 protruded mass, that is, the selvages of the great dyke ; which, if the 

 original mass were granite, would thus appear composed of an axis of 

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

 further 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 intumescent mass by the pressure and resistance of the 

 overlying beds, may be expected to carry their wavings still further, 

 and at the throat of the fissure where the squeeze and jam of the 

 protruded matters must be at its maximum, to occasion those enor- 

 mous and repeated zigzag foldings of the laminated beds, so fre- 

 quently observed in mica- and chlorite-schists in such positions. 



Meantime another influence would be similarly affecting the over- 

 lying 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 frac- 

 tured into cliffy masses, partly broken up into breccias and conglo- 

 merates by this movement ; but the softer beds, especially those 

 which were saturated with water (perhaps even yet under the sea), or 

 which contained interstratified beds of silt, shale, or clay, permeated 

 with water, would glide laterally away from the axis in extensive 

 land-slips, and be wrinkled up into vast foldings under the intense 

 pressure compounded of their own weight, and that perhaps of por- 

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

 similar to the contortions produced in the more crystalline laminated 

 rocks by the violent squeeze which accompanied their protrusion. 

 It may even be difficult to draw a line between the effects of these 

 two replicating and fracturing forces. But, together, they seem to 



