634 ALLUVIA. 



and found in Hertfordshire, ought probably to 

 be referred to the second subdivision, under 

 which they would form a variety C, cemented by 

 quartz. But as the real origin of this substance 

 does not appear to have been ascertained, I have 

 preferred leaving it in doubt for future enquiry, 

 although enumerated in the mean time, under 

 the head of the superior sandstones. 



