1867.] MAW TEETIAEY WHITE CLATS. 393 



the Lignite-formation of Bovey Tracey," published in the Philo- 

 sophical Transactions, make reference to the probable derivation of 

 the deposit from the degradation of the Dartmoor granite. This 

 inference seems to be due more to the geographical proximity of the 

 granite to the clays of the Lignite-formation than to any more certain 

 evidence. 



The present areal ontHne of the deposit, and the surface-contonr of 

 the country, may perhaps give the Lignite -formation a more local 

 aspect than it really possesses ; and the occurrence of beds of similar 

 physical character and age, far removed from the source of granitic 

 materials, would seem to throw doubt on the suggested local origin 

 from the granite of Dartmoor. 



Furthermore the present geographical limits of the deposit may 

 have scarcely any relation to its original extension. It is inter- 

 sected by considerable faults ; and the beds are occasionally disposed 

 at high angles, which have no relation to the present surface-con- 

 tour ; and it seems probable that they may be but a remnant of the 

 original formation, which has been protected from denudation in 

 the Bovey Yalley. 



At the time of the deposition of the white Tertiary clays the 

 chalk must have more completely covered the older formations 

 than at present, and shrouded them over from being sources of 

 supply for the Tertiary deposits ; and the geographical distribution 

 of the white Tertiary clays, which are either superimposed on, 

 or in close proximity to the chalk, suggests a derivation from it 

 rather than from the granitic rocks; indeed nothing resembhng 

 the Tertiary clays in physical aspect occurs far removed from the 

 chalk, except the white clays before referred to, forming isolated 

 deposits on the Mountain -limestone ; and these appear to be the re- 

 sult of local subaerial dissolution analogous to the process suggested 

 for the derivation of the white Tertiary clays from the chalk. 



Another point to be noticed is, that Kaolin (the result of the 

 decomposition of felspar) is perfectly implastic, a feature opposed to 

 the character of the white Tertiary clays, the chemical composition 

 of which favours a derivation from the chalk rather than from 

 the decomposition of the felspar of granite ; and, furthermore, felspar 

 would not provide the proportion of alumina to silica found in the 

 clays*. 



* The inference that might be drawn from this must, however, be received 

 with some caution, as Kaohn, which has certainly been derived from felspathic 

 rocks, ahnost invariably contains a larger proportion of alumina to siHca than 

 that existing in the felspar of the parent mineral. The loss of a part of the 

 silica seems difficult to account for, as, on the decomposition of the felspar, 

 both the silica and alumina would be expected to remain associated in a state 

 of impalpable subdivision not capable of mechanical separation. It may, how- 

 ever, be explained on one of the two following hypotheses. (1) The silica may 

 have been partly segregated into crystalline concretions, and removed in the 

 separation of the granitic quartz from the fine Kaolin ; or, which seems to be 

 more probable, (2) it may have entered into a soluble combination with the 

 alkali of the felspar, and in this condition have been washed out, leaving the 

 alumina with a smaller proportion of silica than that associated with it as 

 felspar. 



