392 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 19, 



forms of the iron and magnesia are not distinguished in the analyses, 

 the whole of them are included in the following average composition 

 of the insoluble matter in the chalk. 



The whole of the eight analyses (namely, five of chalk and three 

 of chalk-marl) give the following result : — 



(q) Alumina and iron 21"13 



Magnesia 4-63 



Silica 74-24 



100-00 



and taking the five specimens of chalk by themselves, omitting the 

 three chalk-marls, 



(r) Alumina and iron 53*76 



Magnesia 8-95 



Silica 37-30 



10000 



As the iron and alumina are not given separately in all the analyses, 

 it has Jbeen necessary to state them together in these averages ; but 

 it must be borne in mind that a part of the iron and magnesia 

 must be deducted (as being soluble in carbonic-acid water) in com- 

 paring them with the composition of the clays ; when this is done 

 it will be found that there is a remarkable resemblance. The 

 average of the five chalks and three chalk-marls gives somewhat 

 more silica than the average of the clays, and the five chalks taken 

 separately somewhat less. 



It must be observed that the chalk-marls contain a great propor- 

 tion of silica, which affects the general average rather dispropor- 

 tionately, as the three examples of chalk-marls to the five of 

 chalk would be greater than the actual mass of the marl in com- 

 parison with the actual mass of the chalk. The whole series of 

 eight analyses averaged with the analyses of the five chalks taken 

 separately (which would represent the marl in proportion to the 

 chalk as 3 to 10) would be a fairer comparison. This mode of esti- 

 mating gives — 



(s) Iron and alumina 37*44 



Magnesia 6*79 



Silica 55-77 



100-00 



which, after deducting a portion of the iron and magnesia as solu- 

 ble, gives a result closely corresponding with the average composi- 

 tion of the clays (p) — namely, in round numbers, 60 per cent, of silica, 

 30 per cent, of alumina, with a little iron, magnesia, &c. When it 

 is considered how extremely the individual examples of both clays 

 and chalks vary, so close a correspondence, on an average of a small 

 number indiscriminately selected, is remarkable. 



The supposed Derivation of the White Tertiary Clays from 

 Granite. — -Messrs. Pengelly and Heer, at p. 9 of their Memoir "- On 



