ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 309 



6. The movements which first gave rise to the mineral 

 springs would open many cavities in which the first deposits from 

 them could be formed, the earliest of all being stanniferous, 

 followed by the sulphides of copper and iron; still later by 

 sulphides of lead and zinc, and, perhaps, last of all, by products 

 resulting from the oxidation of these sulphides. 



7. As regards the period of these granitic eruptions, our 

 best evidence exists on the flanks of Dartmoor. Delabeche was 

 the first to show that the Dartmoor granite has broken through 

 lower carboniferous rocks, and that the Permian conglomerates 

 of South Devon contain fragments of granite, elvan, schorl-rock, 

 and lime-stone. In other words we may say that the eruptions 

 were post-lower carboniferous and pre-permian, probably of 

 about the same age as the great basaltic intrusions of the 

 English coal-fields. The rocks which surround the granite 

 masses in Cornwall are all of much older date than the carbon- 

 iferous period, so that the evidence westward is not quite so 

 clear, nevertheless, the phenomena of all the various granite 

 bosses are so similar and their mineral composition is so nearly 

 identical, that it is natural to conclude that they are in fact 

 portions of the same deep-seated mass, and that all made their 

 way through the strata about the same time. 



8. The first elevatory movements of the granitic substratum 

 began at a considerable depth below the then existing surface, and 

 while the over-lying stratified rocks were much depressed below 

 the sea level. The actual elevation may not have been great, for 

 after they had been injected, first by veins of granitic matter, 

 and then by dykes of felsitic matter of nearly the same ultimate 

 chemical composition, they were broken through at various 

 points, and eruptive material, miles in thickness, was piled up 

 over the stratified rocks which surrounded the openings, as well 

 as over the openings themselves. 



9. This great accumulation of eruptive material was 

 naturally followed by a period of great depression, preceded 

 and accompanied by an enormous amount of denudation. The 

 Permian conglomerates of South Devon are results of this 

 denudation, their colour being' due in aU probability to the 

 ferruginous contents of the denuded rock material, aided by the 



