W. A. E, Ussher — Historical Geology of Cornwall. 79 



Forest Clay of Devon on the tin ground of Cornwall ; to this again 

 must be added the period in which the forests grew ; to this a further 

 addition must be made of the time during which the entire country 

 was carried down at least 70 feet vertically by a subsidence so slow 

 and tranquil and uniform that it nowhere throughout the area of 

 Western Europe and the British Islands disturbed the horizontality 

 of the old forest soil ; and finally we must also add the time which 

 has elapsed since — a time which of itself, thanks to the description of 

 St. Michael's Mount by Diodorus Siculus, we know certainly ex- 

 ceeded 2000 years, and which the volume of the stratified deposits 

 overlying the forests, as well as the amplitude of the existing fore- 

 shore, warrants our believing exceeded it by a very large amount." 



Conclusion. — If the word Cassiterides, in the writings of Strabo, 

 Posidonius, and Diodorus, refers to the Scilly Isles, and if they have 

 also been mentioned by Dionysius Alexandrinus under the name of 

 Hesperides, the quotations from these authors would imply the 

 following consequences. 



First, — That tin must have been obtained in the Scilly Isles as 

 they then existed. 



Secondly, — That, as no productive tin veins or signs of old work- 

 ings are found on these islands, such workings must have been 

 carried on in districts now submerged, at a time when the number 

 of the islands (allowing a considerable margin on the score of 

 insignificance in Strabo's account) was much less than at present, 

 and when the flats between the islands of Tresco, St. Mary, and St. 

 Martin (as may reasonably be inferred from Dr. Borlase's descrip- 

 tion), were dry land at high-water and above the level of spring- 

 tides. 



Thirdly, — That the Channel Islands were not insulated in Diodorus' 

 time ; for, if they were, he would hardly have alluded to the Scilly 

 Isles as nearest to Iberia. This accords with Mr. Peacock's views as 

 to their more recent insulation. 



Fourthly, — From Alexandrinus' account, must we not suppose 

 that the inhabitants of the islands were a colony from Spain in his 

 time, and either supplanted the original inhabitants alluded to by 

 Dr. Borlase, or were themselves succeeded by a British race, 

 addicted to Druidic rites ? 



Notwithstanding, I am inclined to think that the word " Cassi- 

 terides " was indiscriminately used for the Scilly Isles and Land's 

 End District, 1 owing to the imperfect navigation of those eaxdy days 

 of naval commerce. 



Diodorus's description of the inhabitants and mineral wealth of 

 Belerium would apply rather to a district of that name than to an 

 individual promontory, and it does not seem improbable that the 

 name of one of its most important headlands should be indiscrimi- 

 nately applied to the whole stanniferous district of the Land's End. 

 If, as Mr. Peacock supposes, the Northern Channel Islands are 

 spoken of by Diodorus as neighbouring islands with reference to 

 Ictis, one can scarcely agree with him in disposing of the claims of 

 1 T. E. G. S. Com. vol. vii. p. 153. 



