OlllGIN AND DEVELOPMENT OF ORE-DEPOSITS. 369 



The Great Flat Lode described by Dr. Poster in 1878, is a 

 fine example of a capel tin lode.* The microscopical structure 

 of certain tin capels was fully described and illustrated in the 

 author's papers published in 1 880-82. f 



The constant association of schorl and tin has been already 

 referred to. Indeed tin is scarcely ever present without schorl, 

 although schorl very often occurs where there is no tin. 



Tourmaline schist is rarely quite free from tin, but many of 

 the thin films of schorl between the beds at the Parka mines 

 appear to be absolutely devoid of tin. Here the schorl occurs 

 in thin knife-blade films, absolutely black, but fading gradually 

 away to red yellow or white at a short distance from the fissure. 

 In other places quite near "floors" of tin occur in great 

 abundance. 



Sec. 13. — On the localisation of Mineral Substances in the West of 

 England Mining District. 



The contrast between the vast quantities of alkalies present 

 in the constituent minerals of the crystalline rocks, (and in the 

 eruptive rocks which appear to have been derived from the 

 crystallines by more or less complete fusion), and the small 

 proportions existing in the rocks of the sedimentary series, is one 

 of the most noteworthy in the whole range of chemical geology. . 

 A similar rarity of alkaline constituents characterizes the great 

 bulk of our mineral deposits, whether stratified beds, segrega- 

 tions or fissure-veins. 



The alkaline constituents now so abundant in the seas and 

 in vegetation have probably been derived in great part from the 

 crystalline and eruptive rocks by the agency of ascending 

 thermal currents (springs), as suggested by Dr. T. Sterry Hunt 

 in his crenitic hypothesis, J and there is great reason to believe 

 that the mineral deposits themselves have been derived from the 

 same primary source, as we have endeavoured to show in the 

 foregoing sections. 



* Quart. Journ. Geol. 8oc. 



fOn Cornish Tin Stones and Tin Capels, Min. Mag., Vol. 4 and 5. 



X Mineral Physiology, &c., 1886, p. 132, et. seq. 



