98 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 2, 



sure, perpendicular to the cleavage at each place. The curve below 

 shows the outline of a mass below the surface whose expansion or 

 elevation would produce the pressure which we know to have acted 

 on the mass above. 



This is the form which a fluid mass would assume if forced up- 

 wards through a fissure below the crust of the earth coinciding in 

 direction with the axis of the area of elevation. As beds of a slaty 

 character are usually found in close connection with igneous rocks, 

 we may refer the elevation to the upheaval of a mass of fluid igneous 

 matter. The area over which the effect of the upheaval has extended, 

 is obviously bounded by the lines of vertical cleavage. 



It has also been shown, that the compression of the slaty mass 

 was compensated by its expansion in the direction of the dip of the 

 cleavage^ but that no change was observed in the direction of the strike 

 of the cleavage. The explanation of these laws is easy, now that 

 the direction of the cleavage and its position relatively to the eleva- 

 tion of the area are ascertained. The elevation of a mass of rock 

 into a curve by increasing the breadth of the surface of the area, 

 would give room for the expansion of the mass in the direction of 

 the curve, in the proportion of the length of the arc thus formed to 

 its chord ; and the curve by the terms of the proposition represents 

 the dip of the cleavage. But as long as the elevation continued 

 uniform over a straight axis, nothing would occur to weaken the re- 

 sistance to the expansion of the mass in the direction of the axis, 

 which is, as already shown, the direction of the strike of the cleavage. 

 If the elevation should prove sufficient to break up the surface, the 

 fissures would be parallel to the boundaries of the area; and when 

 the mass was once broken in longitudinal fissures, its power of re- 

 sistance to an expansive force would be farther diminished in the 

 direction of the dip, but would still remain the same along the strike. 



These considerations relate to the case of a longitudinal area of 

 elevation, or as Mr. Hopkins has appropriately named it, a case of cy- 

 lindrical elevation*, because these are the only cases in which I have 

 had an opportunity of studying slaty rocks. It would be very in- 

 teresting to find a case where slate had been formed round a conical 

 elevation, and to check the accuracy of the laws deduced from the 

 cases considered, by observing the modifications required to adapt 

 them to new circumstances. 



Depression of an area at some period posterior to its elevation and to 



the cleavage. 



There are several circumstances which tend to show that the centre 

 of an elevated district may have somewhat sunk down again, after 

 the completion of the original elevation and of the cleavage. Such 

 a sinking is probable, in the first instance, from the relaxation of the 

 central pressure when the igneous matter was allowed to escape 

 through the fissures formed at the sides, and again at a later period 



* Abstract of a Memoir on Physical Geology, p. 12. 



