314 Notices of Memoirs — The Geological Survey. 



occupation of tlie country. In the second place, it was recognized 

 that in many various ways these surface-deposits had a direct and 

 vital influence upon the welfare of the population. In agriculture, 

 in water-supply, in questions of drainage, and of the location of 

 dwellings, it was seen that a knowledge of the soils and subsoils, 

 and of the formations from which these are derived, was of the 

 utmost practical importance. It was therefore determined that 

 henceforth the Geological Survey should not only pourtray the 

 lineaments of the solid earth, but trace out the drifts and other 

 surface-deposits which, like a garment, overspread and conceal them. 

 It was impossible at first to go back over the ground where the 

 surface-geology had been omitted. But it was arranged that when 

 the whole country had once been mapped those tracts should be re- 

 examined wherein the superficial deposits had not been surveyed. 

 And, in the meantime, over all new areas the survey was made 

 complete by tracing out simultaneously both the surface-deposits and 

 the older rocks below them. 



The Drift Survey of Wales, and of those parts of England where 

 the superficial deposits were not originally mapped, now occupies 

 the time of a considerable part of the English stafi". In Ireland, 

 also, those tracts where the peat and some other superficial deposits 

 were not delineated are now having this omission remedied. In 

 Scotland the drifts are all mapped at the same time with the rest of 

 the geology. 



As an illustration of the detail into which the mapping in this 

 department has been cai-ried, it may be mentioned that under the 

 single term " alluvium " we now discriminate and indicate by 

 separate signs and colours a large number of distinct deposits. 

 Thus, there is a group of fresh-water alluvia, beginning with the 

 present flood-plains of the rivers and rising by successive terraces 

 to the highest and oldest fluviatile platforms. Deposits of peat 

 are separately traced, and tracts of blown sand are likewise mapped. 

 Another series, consisting of marine alluvia, ranges in position and 

 age from the mud of modern estuaries and the sands of flat shores 

 exposed at low water, through a succession of storm -beaches and 

 raised beaches, up to the highest and most ancient marine terraces 

 100 feet or more above the present level of the sea. Regarding the 

 origin of some of the high-level gravels, there is still much 

 uncertainty, but the Survey has taken the first necessary step for 

 their ultimate explanation by carefully tracing their distribution on 

 the ground. 



But the most abundant and complex group of superficial deposits 

 is that which may be classed under the old name of Glacial Drifts. 

 These have been mapped by the Survey in detail, and much of the 

 progress of glacial geology in this country has been due to the 

 sedulous investigation thus required. The ice-strias on the solid 

 rocks have been observed over so much of the country, that maps 

 may now be constructed to show both the march of the main 

 ice-sheets and the positions of the later valley-glaciers. The various 

 boulder-clays have been mapped, likewise the sands and gravels. 



