206 Dr. Riclietts — On Fissures, Faults, Contortions, etc. 



Observation thus proves that changes in the dip of strata, contor- 

 tions, and cleavage, have been induced by lateral pressure, often 

 propagated in the same direction over extensive areas, and in innu- 

 merable instances the same force has caused both the contortion and 

 cleavage. 



Sir Charles Lyell, in 1850, suggested that to some extent flexures 

 might be caused by the subsidence of mountainous masses beneath the 

 floor of the ocean, which, sinking for miles vertically, must occasion- 

 ally be forced to pack themselves into smaller spaces than they 

 previously occupied.^ Depression must necessarily cause compres- 

 sion of the subsiding portion, until a certain depth is reached, but as 

 it acts in every direction, the contortions must be irregular, unless 

 there exists some cause to give that definite tendency which they so 

 frequently assume. The chief causes capable of producing this 

 effect appear to be valleys and faults. 



Valleys at their seaward extremities become gradually submerged 

 and filled up with sedimentary matter brought down by rivers. If 

 this depression continues, compression must be induced, and the un- 

 consolidated material, confined as it were between walls of hard 

 rock, would have the direction of the pressure applied to it 

 determined by that of ancient submerged and fiUed-up valleys. 

 Therefore the hollows and ridges of the contortions would extend 

 parallel to the course of the valley ; the flexures being greater near 

 the sides, in consequence of the resistance afl"orded by the harder 

 rock. The greatly contorted Silurian strata to the west of the 

 Malverns may be cited as an example. 



As cleavage is dependent on similar pressure, so likewise it 

 would be induced under the same cii'cumstances if the conditions 

 are favourable. To such an origin I would attribute the cleavage 

 as well as the contortions of the Wenlock rocks, at a distance of four 

 miles to the north and the same to the south of Llangollen ; these 

 strata appear to be a deposit in a pre-Wenlock valley. 



The presence of dykes, or other masses of intrusive volcanic rock, 

 by forming points of resistance, have, upon the occurrence of similar 

 changes of level, also influenced the formation of contortions. 



A more powerful agent in influencing the degree and direction of 

 pressure is the occurrence of faults ; and it is generally recognized 

 that their presence has " an intimate connexion with those foldings 

 which constitute anticlinal and synclinal axes." But their forma- 

 tion must be attributed to a different movement from that by which 

 faults have been produced. The wedge-shaped masses, which have 

 fallen down on the formation of faults, have b}' so much enlarged 

 the area they occupy- as the width of the angular portion is inci-eased 

 from the original width at any point previous to the dislocation; 

 but there would be no difference, or very little, in the angle at 

 which the depressed portion lies from that which it previously 

 occupied. 



As the space occupied by the occurrence of faults, whether they 



^ Presidential Address, Quart. Journ. Geol. Sue, vol. vi., p. liv. 



