114 ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 



tachylite. Tachylite is unknown in the West of England, but 

 many of the intrusive rocks of the East of Cornwall, although 

 now much altered, appear to to have been basalts. 



(6). Plutonic rocks are such as have been consolidated 

 from a melted or partially melted condition at considerable 

 depths beneath the surface. Granite and gabbro are holocrystal- 

 line, i.e. they are composed entirely of crystals of various minerals, 

 which have been formed, if not simultaneously, yet very nearly 

 so ; felspar-porphyry, quartz-porphyry, and a great many other 

 rocks consist of a ground-mass either of minute crystals or of 

 crystalline particles, in which are distributed larger crystals of 

 distinct minerals. Such a structure is porphyritic. A third 

 condition is what is known as micro-crystalline or crypto-crystalline, 

 in which the whole substance consists of minute but ill-defined 

 crystalline particles, as in the case of the elvan dykes of Trelaver 

 and St. Dennis Downs. There is, however, reason to believe that 

 this is not an original structure, but rather that it has been 

 subsequently developed from a vitreous condition of the rock. 



Rock-change in relation to mineral deposits. With rocks 

 originating in so many different ways, under widely different 

 conditions, and with their materials drawn from such very 

 different sources, we may expect that the subsequent develop- 

 ments will also be various. We are now in a position to study 

 some of the developments, in so far as they relate to ore-deposits 

 and to their containing rocks. 



The changes which are continually going on in rock-masses 

 from the first moment of their formation are of great importance 

 to the miner. The object of the present writer is to summarise 

 and classify such observations, whether original or made by 

 others, as are specially related to the origin and development of 

 mineral deposits in the West of England — a region notable for 

 the variety and extent of its rock- changes. 



The chief directions in which these changes of rock operate 

 in favour of the miner are the three following, viz : in preparing 

 convenient sites or resting-places for the minerals, in making 

 channels by which they may be brought within his reach from 

 below, and in concentrating such valuable substances as are 

 sparsely distributed to such an extent as to be worth working. 

 Many examples of each of these lines of operation will be given 



