312 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



13. Since the intrusion of the elvans, and perhaps for 

 long periods before those intrusions, there is no evidence of 

 subsidence or marine deposit in this West of England region 

 until we come to the Tertiary period, when there was a con- 

 siderable depression for a long time while the Pliocene beds of 

 St. Erth and the Bovey Tracey clays and lignites were being 

 deposited, and perhaps, too, some of the high level gravels 

 of West Cornwall. Since then, there have been several rather 

 considerable oscillations, but none of them of really great extent. 

 During all those seons the rocks were being denuded away, so 

 that we now see the roots of the old mountain chain which 

 once extended from Dartmoor to the Scilly Isles, and which may 

 have been as high as the Alps or even as the Andes.* 



It is a trite observation among geologists that the poet's 

 "solid ground unchanged for age" does not exist. "Where grew 

 the trees, there rolls the flood." It is plain that in this district, 

 as in all others, changes —chemical, physical, and mechanical 

 have been going on from the earliest times, and are still in 

 progress. There is constant decay and constant re-formation, with 

 a gradual development from minerals of less to those of more 

 permanent character ; felspars are converted into kaolin, micas 

 into schorl ; olivines, augites, and hornblendes into serpentine ; 

 sulphides and carbonates into oxides, while alkalies and other 

 soluble salts are taken up by plants, or carried down to the 

 "changing changeless" sea, on whose "azure brow" "time 

 writes no wrinkle " while she gnaws away the land in lines of 

 cliS to form the great "planes of marine denundation," on 

 which new lands are to be subsequently deposited. 



* Could we see the roots of the Alps, it is probable we should find many 

 mineral veins there. 



