60 TOWN GEOLOGY. [n. 



more probable. He ought also to have symptom e, and 

 so forth; and as I find successively each of these 

 symptoms which are proper to A, my first guess will 

 become more and more probable, till it reaches practical 

 certainty. 



Now let us do the same, and say If this strange 

 dream be true, and the lowlands of the North were 

 once under an icy sea, ought we not to find sea-shells 

 in their sands and clays ? Not abundantly, of course. 

 We can understand that the sea-animals would be too 

 rapidly covered up in mud, and too much disturbed by 

 icebergs and boulders, to be very abundant. But still, 

 some should surely be found here and there. 



Doubtless; and if my northern-town readers will 

 search the boulder-clay pits near them, they will most 

 probably find a few shells, if not in the clay itself, yet 

 in sand-beds mixed with them, and probably underlying 

 them. And this is a notable fact, that the more species 

 of shells they find, the more they will find if they work 

 out their names from any good book of conchology of 

 a northern type ; of shells which notoriously, at this 

 day, inhabit the colder seas. 



It is impossible for me here to enter at length on a 

 subject on which a whole literature has been already 

 written. Those who wish to study it may find all that 

 they need know, and more, in Lyell ; s " Student's 

 Elements of Geology," and in chapter xii. of his 

 " Antiquity of Man." They will find that if the 

 evidence of scientific conchologists be worth anything, 

 the period can be pointed out in the strata, though not 

 of course in time, at which these seas began to grow 

 colder, and southern and Mediterranean shells to dis- 

 appear, their places being taken by shells of a temperate, 



