200 GEOLOGY OF THE ISLE OF WIGHT. 



siderable extent are quite vertical (see PI. XIII.). 

 The parallelism of the cretaceous strata, and the 

 absence of sand, gravel, and other coarse detritus, 

 denote that the deposition of these calcareous sedi- 

 ments took place in tranquil water, and remote 

 from sea-cliffs and shores. Where a large extent 

 of the cliff is exposed, the face of the chalk is seen 

 to be traversed by fissures or joints at right angles 

 to the planes of stratification ; some are vertical, 

 others diagonal, and crossing in various directions, 

 divide the entire mass into irregular portions ; 

 in some localities these partake of a rhomboidal 

 form.* These fissures have evidently been pro- 

 duced after the consolidation of the strata in which 

 they occur, for where they traverse the horizontal 

 layers of flint nodules, some of the latter are often 

 displaced ; and very commonly a fault, or inter- 

 ruption of the parallelism of the opposite sides of 

 the chasm, has taken place, either from the elevation 

 or the subsidence of one of the separated masses. 

 In many instances the fissures are filled up with 

 flint, which occupies the crevice wholly, or partially, 

 throughout its whole extent, forming large sheets 

 of pure silex, varying in thickness from that of 



* Mr. Webster particularizes the large chalk-pit on the south side of Arre- 

 ton Down near the top, not far from Ryde. In this quarry the Btrata are 

 inclined at an angle of 80 deg. ; and both the tabular flints and the nodules 

 are completely shivered. 



