THE CHALK 51 



The extensive downs in the centre of the Island 

 are largely overspread with angular flint gravel 

 similarly formed to that of St. Boniface. Of other 

 beds of gravel, which have been washed down to a 

 lower level by rivers or other agency we shall have more 

 to say later. 



The Chalk strata in the Isle of Wight are of great thick- 

 ness. In the Culver Cliff there are some 400 feet of flint- 

 less Chalk (Lower and Middle Chalk), and then some 

 1,000 feet of chalk with flints. There is some variation 

 in the thickness of the strata in different parts of the 

 Island, and the amount of the Upper strata, which has 

 been removed by denudation, varies considerably. The 

 average thickness of the white chalk in the Island is about 

 1,250 feet* Including the Lower Chalk, the maximum 

 thickness of the Chalk strata is 1,630 ft. 



The divisions of the chalk we have so far considered 

 depend on the character of the rock : we must say 

 a word about another way of dividing the strata. 

 It is found that in the chalk, as in other strata, 

 fossils change with every few feet of deposit. We may make 

 a zoological division of the chalk by seeing how the fossils 

 are distributed. The Chalk was first studied from this 

 point of view by the great French geologist, M. Barrois, 

 who divided it into zones, according to the nature of the 

 animal life, the zones being called by the name of some 

 fossil specially characteristic of a particular zone. More 

 recently Dr. A. W. Rowe has made a very careful study 

 of the zones of the White Chalk, and is now our chief 

 authority on the subject. The strata have been grouped 

 into zones as follows : 



"1,472 ft. at the western end of the Island, 1,213 ft. at the eastern. 

 Dr. Rowe's measurements, 



