468 On tlie Agricultural Geology of England and Wales. 



progress of erroneous conclusions from imperfect data which have 

 been corrected by advancing knowledge, derived from more ex- 

 tended observation. Geology has not been more exempt from 

 these than the exact sciences of astronomy and chemistry. By 

 one of those hasty generalisations, all the detrital deposits above 

 described, together with all gravel beds in all parts of the worlds 

 including many tertiary strata of much older date, were once on 

 insufficient evidence assumed to be contemporaneous and monu- 

 ments of the Noachian deluge. It has since been assumed, on 

 evidence equally insufficient, that they belong to different epochs. 

 We viev/ them as the results of a connected series of operations, 

 during one zoological epoch — an epoch of considerable duration 

 — the series of operations being the accumulation of the deposits 

 during the subsidence, and their denudation during re-elevation 

 of the land. If space permitted, it would be easy to show how 

 the distribution of these deposits, the intermixture of foreign and 

 local detritus in some situations, the prevalence of local gravel in 

 others, and the absence of all detritus from some localities, agree 

 with the theory of the action of shore-ice on sinking land, the 

 gradual advance of the coast-line into the interior during sub- 

 sidence, and the transport of local fragments outwards during the 

 period of elevation ; but our object is the application of the sur- 

 face-geology of England and Wales to its agriculture, and we 

 would rather draw attention to the large area which has been 

 covered with these deposits^ the materials of which they are 

 composed, and the height to which they have ascended. 



In the first place, then, it is a general fact, common to the area 

 now under consideration, and to every district which we have 

 examined in Ireland, that the erratic tertiaries consist of a lower 

 and an upper deposit, the former composed of clay, the latter of 

 sand and gravel. The colour of the lower deposit, the till or 

 boulder clay, is blue, brown, red, yellow, or white from inter- 

 mixture of chalk, with various intermediate shades, according to 

 the prevalent colour of the rocks from which it has been derived, 

 W'hich are chiefly those of the neighbourhood. The peculiarities 

 of this lower bed indicate the littoral action of an icy sea, in which 

 it appears, from the observations and soundings of the polar navi- 

 gators, that mud accumulates in situations where sand and shingle 

 would be deposited in other seas. 



The sand and gravel of the upper erratics partake more of the 

 characters of ordinary tertiary strata ; but they possess some pe- 

 culiarities, such as the occasional presence of masses of frag- 

 mentary matter unabraded and unmixed with other detritus, and 

 the presence also of blocks of large size, derived from far distant 

 rocks. Both of these appear to require some buoyant material 

 for their transport. The difference between the lower and upper 



