68 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



gists call for many thousands of years, because human indications 

 have been found buried in detrital accumulations at considerable 

 depths below the present surface. Such a conclusion would 

 seem to be but little warranted in view of the evidence we have 

 of rapid accumulation in our lower valleys. Thus, a crucifix, 

 certainly an article of very modern date from a geological point 

 of view, was found at the bottom of Carnon Stream Works in 

 1812, at a depth of about 30 feet below the bed of the river*'. Still, 

 the antiquity of the gravel formations must be vastly greater 

 than that of their first working by man, and it has been shewn 

 conclusively by Mr. "Worth and others, that tin-working has gone 

 on in Cornwall, for certainly 3000 and possibly more than 4000 

 years. f Our Museum is probably richer in objects of interest 

 illustrating the early tin-streaming industry than any other 

 museum in the world. 



Sec. 4. Mineral Associations. 



The detrital tin-ore of the West of England occurs as crys- 

 tals, pebbles, sub-angular masses, and water-worn grains of 

 cassiterite, sometimes associated with similar fragments of iron- 

 pyrites and other " heavy " metallic minerals, and occasionally 

 with small particles or even small nuggets of gold. The cassiter- 

 ite is sometimes attached to fragments of granite or slate, or to 

 pieces of quartz, felspar or other veinstone, but more commonly 

 it is free from such attachments. In any ease, it forms but a 

 very small proportion of the " tin-ground," rarely exceeding 

 five per cent, of its mass, and often not so much as one-tenth of 

 one per cent. 



Whatever may be the thought of the hypothesis which has 

 once and again been put forward that some of the tin consists of 

 concretions formed in situ, we must in the main conclude that 

 the gravels and their included tin particles have been formed by 

 the crumbling and wearing away of rocks enclosing tin-veins. 

 We are led to this conclusion by comparing the tin-associates in 

 the gravels with similar associations in known veins, which in 



* Barham, iJep. Roy. Jnst. Corn., p. 6. 



f Worth, Antiquity of Mining in the West of England. 



,, Progress of Mining Skill. 



„ Ancient Mining Implements. 



