392 GEOLOGY OF THE ISLE OF PURBECK. 



tension to which the strata were subjected during 

 their elevation, by the expansion of elastic vapours, 

 or heated erupted rocks, far beneath the surface. 

 For Mr. Hopkins has shown, on mathematical prin- 

 ciples, that tabular masses of rock elevated by a 

 force from below, must have been exposed to two 

 sets of tension, which would produce longitudinal 

 and transverse vertical fractures at right angles to 

 each other; and the direction of the principal 

 fissures, or lines of fracture, in the tracts of country 

 we have surveyed, remarkably coincides with this 

 theoretical deduction. 



Pseudo-volcano. — A circumstance connected 

 with the bituminous property of the Kimmeridge 

 clay, which happened in Ringstead Bay some years 

 since, may here be mentioned. In September 

 1826, at Holworth Cliff, and adjacent to the pro- 

 montory of Whitenore, a spontaneous combustion 

 of the bituminous shale occurred, and burst forth 

 with considerable volumes of flame, and exhala- 

 tions of sulphureous vapours. It continued smoul- 

 dering for many years, and the surrounding clay 

 and shale were burnt into cellular slag. This 

 combustion probably originated in the heat pro- 

 duced by the decomposition of pyrites, with which 

 the strata abound.* 



* See the Memoir on the Geology of Weymouth, &c. Oeol. Trans, 

 vol. iv. 



