100 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 2, 



and then turns down the cleavage forming irregular sheets between 

 the slates, which frequently do not cross the whole thickness of the 

 bed of slate. This has apparently followed upon the irregular gaping 

 of some of the cleavage planes. 



Both in this case and in tlie instances mentioned by Mr. Darwin, 

 the cleavage dipped at a high angle. The opening of the rock along 

 the cleavage would have been impossible as long as the original 

 pressure continued, and, to account for it, it requires a cause exactly 

 opposite to that which produced the pressure, such as the sinking of 

 the elevated area. 



Position of the beds over a given area of elevation, 



I must now consider some theoretical questions connected with 

 the elevation of portions of the crust of the earth, in the hope of 

 finding how to ascertain, from the position of the beds, what is to be 

 considered as one area of elevation, or in other words, of learning the 

 extent of surface over which the present position of the rocks is to 

 be regarded as produced by a single cause. This is not irrelevant 

 to my principal object ; for if it can be ascertained, we shall be able 

 to compare conclusions drawn from the position of the cleavage 

 planes with those derived from the stratification, and thus test their 

 accuracy. In this I shall have to pass over some of the ground on 

 which Mr. Hopkins has laboured so eflSciently ; but as the points at 

 which I am aiming are not exactly those to which his papers are 

 devoted, I shall derive assistance from the general tenor of his argu- 

 ments, and when unable to refer to particular passages for authority, 

 I shall still be benefiting indirectly by his guidance*. 



The districts I have referred to, and of which I wish to explain 

 the phasnomena, belong to the case which Mr. Hopkins has appro- 

 priately termed cylindrical elevation^ namely that where the area is 

 of indefinite length and bounded laterally by parallel lines. Mr. 

 Hopkins has so fully explained all that relates to cross fractures and 

 cross lines of elevation, that 1 shall not touch upon that branch of 

 the subject, but confine myself to the primary lines of elevation, 

 which are those parallel to the boundaries of the area. 



We meet with two series of longitudinal cases of elevation, pro- 

 ducing very difi'erent results on the beds they aflTect. The simplest 

 is that where a mass of igneous matter has broken through the sur- 

 face, as in the Malvern Hills. Here the greatest disturbance is close 

 to the line of eruption : the beds nearest to it are highly inclined, and 

 as we recede from the axis we find them less and less raised, till the 

 effect can no longer be traced : the dip in these cases is from the 

 axis of elevation, unless the violent raising of the beds has caused 

 them to give way, and then we may find minor synclinal and anti- 

 clinal axes parallel to the main line. In these cases we see the 



* Hopkins, Kesearches in Physical Geology, Transactions of the Cambridge 

 Philosophical Society, vol. vi. ; Abstract of a Memoir on Physical Geology ; On 

 the Wealden district and Bas Boulonnais, Transactions of Geological Society, 

 2nd Series, vol. vii. p. 1. I have principally consulted the last two, which are ad- 

 dressed to geologists who are not mathematicians, and have had them always before 

 me in writing tlie following pages. 



