DILUVIUM. DRIFT AND ERRATIC BLOCKS. 



751 



oelongs 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. 



DILUVIUM. DRIFT AND ERRATIC BLOCKS. 



NDER the general denomination 

 of diluvium, drift, boulder for- 

 mation, erratic block groups, or 

 osseous breccia phrases which 

 are more or less vague and un- 

 satisfactory diversified deposits 

 pp 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 eleva- 

 tions; distributed over forma- 

 tions of all ages, either consti- 

 tuting the visible superficies, or 

 thinly covered with the turf and 

 cultivable soil, and forming, as 

 it has been happily expressed, the loose vestments of the globe. The greater part of the 

 drift a term which is sufficiently accurate for a very general view of the subject is com- 

 posed of dark tenacious blue or red clay, sand, and gravel, varying from a slight thickness 

 to that of several hundred feet, the usual order of the series being first and lowest, the 

 gravel, then clay, and then sand. The gravel and clay show different degrees of commi- 

 nution, and appear tumultuous accumulations, their respective parts being mixed together 

 in the manner that violent currents of water would be likely to produce. The sand 

 evinces a much more quiet deposition ; and, if traceable to the same cause, it belongs to 

 a different age. 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 era of Mr. Lyell. It is distinguishable from the older tertiary depo- 

 sits by its confused aggregation and general unstrtitified character ; and from accumulations 

 proceeding at the present period, by its occurring at every altitude attainable by moun- 

 tians 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, Massachusseti 



