580 MANUAL OF NATURAL HISTORY. 



gonites" or fossil fruits of the genus Cham, with 

 spirally twisted valves, are also common. 



IV. — Glacial, or Post-Pliocene Group. 



This group, sometimes called the Ancient or Dilu- 

 vial Drift, consists of deposits still in course of 

 formation ; beds of rivers, lakes, peat-bogs, coral 

 limestones, volcanic ejections, and calcareous de- 

 posits from mineral springs. These results are com- 

 monly ascribed to the unusual operations of water, 

 or by the passage of diluvial waters over the sur- 

 face of the earth, thus accounting for the gravel, 

 sand, and clay, with boulders, or rounded masses, 

 and water- worn transported materials. What are 

 termed " erratics " also occur in this group ; they con- 

 sist of large, angular masses of rock, distinct from the 

 rounded boulders, and are now believed to have 

 been transported by ice, and so to have preserved 

 their angular forms : they occur only in extra-tro- 

 pical regions. The scratches or grooves on the 

 surface of rocks of this epoch are accounted for by 

 the pebbles moving with, and immersed in, the beds 

 of boulder-clay, which once covered the rocks j the 

 movements being produced by landslips, and the 

 descent of semi-fluid mud down the sides of moun- 

 tains and along valleys. 



In this last deposit of the tertiary formations, we 

 find the remains of many large mammals of species 

 both recent and extinct. Ossiferous Sands and 

 Gravel of the valleys of Great Britain, the conti- 

 nent of Europe, and the river-plains of North 



