§ 10. ERRATIC BOULDERS. 209 



were, in broad streams from mountain chains; in others 

 constituting a series of terraces and platforms, as if ori- 

 ginating in gradual and successive elevations of a sea-coast. 

 The composition of these deposits is as various as their 

 origin. They, are generally made up of the detritus of rocks 

 of all ages, promiscuously intermingled, and varying in 

 lithological composition from fine earth and sand, gravel, 

 and pebbles, to large boulders, and rocks many tons in 

 weight. They sometimes consist of the debris of the strata 

 of the adjacent country, but more frequently of rocks that 

 are situated in very remote regions. 



It will be apparent to the intelligent reader, that depo- 

 sits varying so much in mineral characters and structure, 

 must belong to very different epochs, and have been 

 accumulated under very dissimilar conditions. Some are 

 unquestionably ancient shingles or sea-beaches (see p. 113) : 

 others are sheets of detritus that have been formed alon£ 

 shallow coast-lines, or on the borders of rivers, lakes, deltas, 

 and estuaries : others consist of ridges and mounds of 

 debris heaped up by sudden deluges, or powerful floods 

 that have swept over low tracts of country, or by subaqueous 

 currents, or waves of translation : and others have been 

 produced by the agency of frozen water, either as floating 

 icebergs, or as glaciers. By the nature and state of the 

 shells, bones, and other organic remains, when present, and 

 of the gravel, pebbles, boulders, &c. of which the accumu- 

 lation of drift or alluvium may consist, the origin of the 

 materials, and the age and character of the deposit, may be 

 more or less satisfactorily determined.* 



10. Erratic Boulders. — But there is another class of 

 phenomena which must not be passed over without com- 

 ment. In many countries rounded and angular boulders of 



* Mr. Lyell's "Principles of Geology" should be consulted for a full 

 exposition of this subject. 



P 



