196 Outlines of Geology, 



to have dressed their venison. But although the magnificence of 

 the object is such as to heat the imagination, and impose upon 

 the judgment when considered as a work of art, we must look to 

 some natural operation for the origin and cause of the pheno- 

 mena, and among these none more plausible than the idea of the 

 lake having successively occupied the different heights in the 

 valley of these roads, and having formed them by the continual 

 action of its waters, the edges and boundaries of which they repre- 

 sent ; of such phenomena we shall offer more decided evidence 

 in a future lecture. 



The depositions of gravel so common in the London basin, and 

 more especially upon the north of the metropolis, are very gene- 

 rally admitted to be of aqueous origin, and it has been supposed 

 chiefly to have arisen from the wearing away and demolition of 

 strata once lying above these alluvial matters, and especially from 

 chalk flints rounded by the attrition of those waters which broke 

 down and washed away the chalk that contained them ; there is, 

 however, the most distinct evidence of the more remote original of 

 these beds, for they contain pebbles very unlike those flints now 

 found in chalk, and they often present pieces of granite, quartz, 

 and other matters, not only not existing in the neighbourhood, but 

 very unlikely ever to have formed strata lying above our present 

 chalk beds. Some more extensive, violent, or general cause, 

 therefore, must have operated, as indeed is rendered evident by 

 the inspection and situation of those larger and scarcer masses of 

 rock, evidently rounded and transported, which are called bout" 

 ders ; these were probably much more abundant in former times 

 than at present, having been removed partly for the purpose of 

 building, and partly in clearing land for cultivation, but how they 

 arrived in their present situations, often far distant from their 

 evident sources, and not unfrequently with hills and valleys inter- 

 vening, is a question which can only be answered by supposing 

 what may perhaps be regarded an extraordinary and unwarrant- 

 able stretch of hypothesis; namely, that these boulders were 

 rolled into the places they now occupy, by some tremendous cur- 

 rent and inundation, carrying sand, gravel, and boulders along 



