194 



THE GEOLOGIST. 



feet. The rate of accumulation, though it may often be slow, is very va- 

 riable. A Eoman road in the valley of the Lea was found covered by 

 two feet only of alluvium. Another such road in Cambridgeshire was 

 covered with five feet of alluvium. The entire depth of the alluvium was 

 not, however, ascertained in either case. M. Kozet gives another instance 

 of a Eoman road, which he considers to have been kept in repair until 

 about the eighth century, traversing the valley of the Dheune. Its paved 

 and even surface is now covered by-twelve inches and a half of alluvial 

 soil. A little lower down the valley this alluvium, which is very uniform, 

 has been ascertained to be about thirteen feet thick. This he estimates 

 would have required for its accumulation about 10,000 years. The allu- 

 vial soil reposes there immediately on the so-called diluvium. The rapi- 

 dity with which the alluvial soil will accumulate under favourable condi- 

 tions is often very much greater. In places, thick beds of alluvium and 

 of peat have been formed since the Eoman occupation. Looking at these 

 facts, and at the general fact, that as a rule, in the. valleys of the Sorame 

 and of the Thames, for example, the E#man, British, or Gaulish remains 

 are found at a depth from the surface bearing a considerable proportion 

 to the entire thickness of the alluvium, the probability is, that the com- 

 mencement of the alluvial deposits is not to be carried back indefinitely. 



One reason for believing the accumulation of the silty alluvium of oar 

 valleys to have been more rapid at one time than now is, that these val- 

 leys, left rude and rugged at the end of the Quaternary period, would be 

 subject to more frequent floods until their inequalities were filled up and 

 levelled. Mr. Prestwich concluded by observing that for these and various 

 other reasons he was confirmed in the opinion he expressed in 1859, that 

 " the evidence, as it stood, seemed to me as much to necessitate the bringing 

 forward of the extinct animals towards our own time as the carrying back 

 of man in geological time." In making that observation, he had chiefly 

 in view the distance of time at which the last of the great extinct mam- 

 malia disappeared. If there should have been, between the modern 

 valley alluvia and the latest Quaternary beds, some intervening period of 

 time of which we are ignorant, that distance may be materially prolonged. 

 If, on the contrary, they followed in immediate succession, — and he 

 thought we have evidence that such was the case, for there seems rea- 

 son to believe that some of the large pachyderms still existed at the 

 commencement of the alluvial period, whilst we know that many of the 

 ruminants lived on uninterruptedlv from one period to the other, — he did 

 not, for his part, see any geological reasons why the great extinct mam- 

 malia should not haye lived down to comparatively recent times, possibly 

 not further back than 8000 to 10.000 years. 



"But this only brings us to the threshold of that dim and mysterious 

 antiquity in which first appear those rudely-wrought flints— those evident 

 works of design — those palpable shadowings of man. - Here our chrono- 

 logy fails us altogether. If we look at our broad and long valleys, and 

 then at the comparatively small streams now winding through them, and 

 suppose these streams to have been the same in past times as they now 

 are, we could hardly avoid the conclusion that the time required to pro- 

 duce such excavations with such means must be almost incalculable. But 

 if the view here proposed be correct, it would follow that with rivers so 

 large in proportion to those now occupying the same valleys, with floods 

 of a force now unknown in the same districts, with a cold so severe as to 

 shatter the rocks and to hasten the removal of their debris, we should 

 have, I contend, agencies in operation so far exceeding in power any now 



