402 R. M. Brydone — TJce Origin of Flint. 



irreconcilable with any theory but that of the flinty envelope being 

 practically contemporaneous with the chalk ; the flint veins, some 

 of which are nearly at right angles to the stratification, seem 

 to be irreconcilable with anything like contemporaneous origin. 

 Mr. Richardson states that " there is no difference between the two 

 types of flint " (i.e. the bedded and the vein flint). Chemically and 

 optically that has presumably been established for some vein or 

 veins ; but unless cavities containing powder, or wholly enclosed 

 fossils, are commonly found in vein flint, and vein flint commonly 

 occurs with a thick cortex, none of which things are within my 

 experience, the statement can hardly be accurate as it stands. 



A factor of considerable importance entirely overlooked by 

 Mr. Richardson is to be found in the marl seams so abundant in the 

 flinty chalk. These must be more impervious than the chalk, and 

 would naturally hold up any downward diffusing or draining solution. 

 But the relationship between marl seams and flint rows to which 

 this would naturally give rise on Mr. Richardson's theory, i.e. marl 

 seams coinciding with or just below flint rows, is almost unknown in 

 practice. On the other hand, any advocate of a precipitation after 

 gravitation theory might well appeal to the absolute freedom from 

 marl seams of the great thickness of Thanet Chalk, whose silica, 

 according to him, is now aggregated into the " Three Inch " band, 

 and might also appeal to the disseminated marl in the Lower and 

 Middle Chalk as the reason why no drainage aggregation of silica 

 took place there. Incidentally it may be observed that the soft, 

 spongy, dark-grey flints which are so abundant at Trimingham 

 are characteristic of the marly chalk. 



It is not safe to place much reliance on Mr. Richardson's table 

 (No. 2) of silica contents. He budgets there for no less than twenty- 

 two layers of 3 inches or more in thickness, containing 100 per cent 

 of silica. When it is considered that the unique " Three Inch " 

 band is no more than a layer 3 inches thick containing 100 per cent 

 of silica, it will be seen how misleading an estimate must be which is 

 based on the assumed presence of twenty-two " Three Inch " bands 

 within the zone of M. coranguinum in Thanet (to say nothing of an 

 assumed 12 in. band of solid silica in the zone of Marswpites). He 

 has further budgeted for thirty layers of 3 inches, containing 50 per 

 cent of silica, and therefore each the equivalent of a tabular 1^ inches 

 thick. If he will construct to scale any conceivable section of a 

 layer 3 inches thick, of which 50 per cent shall be flint, he will see 

 at once that such things are extremely rare in the Chalk, ^and that 

 there cannot possibly be thirty of them in the Kent Coast section 

 in addition to twenty-two " Three Inch " bands. 



In his Fig. 4 he shows the mucronata chalk of Dorset as flintless, 

 and in the text he states that in the mucronota zone the flints are 

 generally small and immature. I do not know what his authority 

 is, but I can assure him that at Studland, at any rate, the mucronata 

 chalk of Dorset is by no means flintless, but contains strong rows 



