CHALK. 77 



dug. Lioceramus mytiloides ocours in great profusion towards the 

 upper part of the Middle Chalk. 



The Upper Chalk occupies the whole area of the Downs except 

 the steep slope in which the lower sub-divisions crop out as 

 just described. It consists of a great thickness of white chalk 

 with numerous lines of flints, Towards the base the flints be- 

 come more sparse and grey, and gradually disappear, but below 

 the lowest flint there occur nodules of hard siliceous chalk, havino; 

 the form of flints, but the texture of chalk. This flintless 

 portion of the Upper Chalk varies from 15 to 20 feet in thickness 

 and has the Chalk Rock for its base. 



The line engraved on the map shows the position of the Chalk 

 Rock, but on so small a scale as the one inch scale, especially 

 where the dip is high, represents pretty closely the base of the 

 flinty chalk. 



The flint in the Chalk occurs for the most part as irregularly 

 shaped nodules, but sometimes as tabular layers either coincident 

 with the stratification or filling cracks and joints. Those flints 

 which occur parallel with the bedding, are of a difterent age 

 from those filling the cracks and joints. The former have been 

 derived from siliceous matter, frequently and perhaps in most 

 instances deposited contemporaneously with the calcareous sedi- 

 ment of which the Chalk is composed, around sponges and other 

 organised bodies, the forms and internal structure of which are 

 still preserved. The latter, on the contrary, are of more recent 

 origin, having been carried by percolating water, holding silica in 

 solution, into cracks and joints, where they occur as thin plates 

 of black flint, from | to 1 inch in thickness, frequently separated 

 by a central hollow, or porous grey layer. These subsequently 

 introduced flints are, as might be expected, unfossiliferous, instead 

 of abounding in fossils, as is the case with those of contempo- 

 raneous formation. 



The cracks and joints filled with this secondary flint were not 

 improbably due to the movements which upheaved the rocks of 

 this region. These movements will be shewn in a later Chapter 

 to have taken place at a late Tertiary date. The redistribution 

 of the silica was thus probably in progress after the Chalk and the 

 flints in it had been buried beneath a great thickness of Tertiary 

 Beds, and had assmned their present consistency. There is no 

 reason to doubt that in certain situations the transposition of the 

 silica is still in progress. 



In the parts of the Island where the strata are most hio-hly 

 inclined, the fossils in the more plastic strata, such as the Chalk 

 Marl, are greatl}^ distorted by pressure. The flints also which 

 appear to be whole when viewed in situ, are found on closer 

 examination to be nearly all broken, so that when extracted from 

 the quarry they fall to pieces. The cracks are mostly filled with 

 chalky matter, and the flints themselves appear to have been 

 squeezed into the body of the Chalk, under the influence of the 

 elevatory force by which it has been made to assume its present 

 highly inclined position. These appearances are not observable 



