206 Mr. AiKiN on the Wrekin and on the 



through the marl is a subsidiary argument of no small weight : these 

 tubes, some of which are very thin and scarcely an eighth of an inch 

 in diameter, with a length of twelve inches, are in a position perpen- 

 dicular to the plane of the stratum, which, when this latter is at an 

 angle of 40", causes the coralline tubes to form with the plane of the 

 horizon an angle of 50° ; a situation by no means agreeable to the 

 known habit of this class of animals which always affects a vertical 

 position with regard to the horizon. 



If then it be conceded that these beds have undergone a vertical 

 motion, what remains is to collect the local probabilities relative to 

 each of the two methods, by which, as already described, mineral 

 beds are elevated or depressed. 



The principal argument in favour of motion by depression, is the 

 absence of any unstratified rock between the elevated stratum and 

 that which naturally lies below and in contact with it ; to which may 

 be added the fracture and disturbance of those superincumbent beds 

 which lie on the dip of the elevated stratum. These circumstances, 

 however, are directly the opposite of those which take place at the 

 Steeraway-hill ; for, in the first place, the coal strata that lie upon the 

 limestone crop out with perfect regularity, and nearly horizontal, 

 along the opposite side of the valley, parallel to the hill and not 

 more than two hundred yards from it, a line which, on the hypo- 

 thesis of depression, would be the precise situation of the principal 

 disturbance. Secondly, the beds of limestone and sandstone, which 

 a hundred yards south of the Steeraway are found with an elevation 

 of about 24° and resting immediately on a soft and sandy slate clay, 

 are in the Steeraway itself tilted up at an angle of 40°, with a great 

 mass of greenstone interposed between them and the slate clay. Is 

 it not therefore probable that the greenstone has occupied the situ- 

 ation which it now holds, posteriorly to the formation of the stratified 



