Wilson — On Contortions and Faults, 205 



bone wliicli appears to be given off from the inner side of the man- 

 dible of Fristerodon, nearly opposite the lower d in Fig. 1, PI. XII. 



DESCRIPTION OF PLATES XI. and XII. 



Pl. XI. — Fig. 1. The impression of the ventral face of Saurosternon Bainii, of the 

 natural size, a, the extremity of the tail ; b, the coraioid ; c, the 

 interclavicle or epistema. 



2. A portion of the counterpart, d, the manus ; b, c, as before. 



3. The left foot ; from the counterpart twice the natural size. 



Pl. XII.— Fig. 1. The skull of Prisferodon McKayi, of the natural size. The greater 

 part of the left half of the skull has split off, leaving the left ramus 

 of the mandible {c) exposed, a, the right temporal fossa; i, the 

 orbit ; d d^ the teeth. 

 \a. A tooth, X 2. 



2. A detached ramus of a mandible of Pri&ierodon, viewed laterally, 



and apparently from the inner side. 



3. A similar mandible, viewed from above. 



II. — On the Cause of Contortions and Faults. 



By J. M. Wilson, M.A., F.G.S., etc. ; Fellow of St. John's College, Cambridge; 

 Assistant Master at Rugby School. 



DURING the last few years, in lecturing on geology at Rugby 

 School, I have frequently given an explanation of the causes 

 that produce contortions and faults, which I find, to my surprise, 

 is not given in the ordinary text books, and yet seems to me an 

 extremely obvious explanation. 



The explanation given by Lyell (Elements, p. 64), is contained 

 virtually in the diagram he gives (Fig. 1). I quote however a few 

 words from his book. ''Suppose the mass of rock a, b, c (Fig. 1), 

 to overlie an extensive chasm d, e, formed at the depth of several 

 miles. Now, if this region be convulsed by earthquakes, the fissures 

 /, g, and others at right angles to 

 them, may sever the mass b from 

 A and from c, so that it may 

 move freely, and allow it to sink 

 into the chasm." ^'^' "" 



It is clear from this that Lyell considers faults as caused by 

 subsidence of detached portions of the crust of the earth ; and since 

 a wedge-shaped block could not so subside, the faults must be either 

 vertical or overhanging on one side of the detached portion ; that is 

 faults would *' hade to the upthrow " as often as to the downthrow, 

 which is not the case. 



Phillips, I believe, offers no explanation of faults, but points out 

 the very general law that the plane of separation slopes tinder the 

 depressed portion of the disrupted strata, expressed above by saying 

 that faults " hade to the downthrow." 



Page does not discuss the question. 



Jukes discusses it at considerable length, and his explanation, in 

 some respects, is like my own. He explains, however, the general 



