DRIFT AST) ERRATIC BLOCKS. 



751 



belongs to the same era, the quarries of which abound with innumerable remains of land 

 and river organisms, proclaiming the existence of an ancient lake in this locality in whose 

 bed they were deposited. The volcanic masses of the Rhine, consisting of the Roder- 

 berg, the Eifel, and the " castled crag of Drachenfels," with its associates, forming the 

 Siebengebirge, or group of Seven Mountains, and almost all the vine-clad hills from Bonn 

 to Mayence, on each side of the river, which have been erupted through secondary rocks 

 during the tertiary era, are enduring memorials of some of the great physical changes 

 which have transpired in this attractive region since the general contour of the continent 

 was formed. 



CHAPTER XI. 



DRIFT AND EBEATIC BLOCKS. 



NDER a variety of designations, 

 as the "drift," "boulder-clay," 

 and "erratic-block group" the 

 diluvium of the early geologists 

 diversified deposits are embraced, 

 widely spread over the surface 

 of many countries, but entirely 

 wanting in others ; occurring in 

 valleys, on plains, on plateaux, 

 and at high elevations; distri 

 buted over formations of all ages, 

 either constituting the visible 

 superficies, or thinly covered with 

 the turf and cultivable soil. The 

 Drift a term which is suffici 

 ently accurate for a general view 

 of the subject is composed 

 of, First, a tenacious and compact clay, of blue or red colour, and containing 

 boulders of various sizes distributed throughout its mass ; Second, lying over 

 this, a series of beds of gravel, sand, and plastic or brick-clay. The former appears 

 generally a tumultuous accumulation, the result of some species of violent action ; the 

 latter usually bear the marks of a quiet deposition from water. In the tenacious or 

 boirider-clay, there are occasionally found small beds of water-laid gravel and sand, as 

 indications of times of quiet occurring throughout its deposition. Sometimes the sand and 

 gravel have been consolidated into sandstone and conglomerate by the infiltration of iron or 

 carbonate of lime. The drift belongs to the Newer Pliocene or Pleistocene era. It 

 is distinguishable from the older tertiary deposits by its confused aggregation and 

 general unstratified character; and from accumulations proceeding at the present 

 period, by its occurring at every altitude attainable by mountains in situations where 

 no agency as now acting could have placed it. In many instances its materials 

 have been originated locally; that is, been derived from rocks within a few miles 

 of the spot where they are now found, as appears from the accordance of their 



Erratic Blocks, in Gloucester, 5Iassnci.u;i . 



