109 



ON THE ORIGIN AND DEVELOPMENT OF ORE DEPOSITS IN 

 THE WEST OF ENGLAND. 



Bt J. H. COLLINS, F.G.S. 



rock-t, 



Introduction. 

 General remarks on the origin, formation, and alteration of 



Early condition of the Globe. If we provisionally accept the 

 nebular origin of the earth as a convenient and probable working 

 hypothesis, we may regard it, in its early stages, as a globe of 

 partly liquid, partly solidified rock-magma still mingled with 

 and evolving various gases and vapours, and surrounded with a 

 dense atmosphere of a similar nature. 



As cooling continued, not only would this atmosphere be 

 increased by emissions from the newly-formed globe, but a more 

 or less complete solid shell would at last be formed on the 

 surface of the mainly liquid nucleus. The existence of this shell 

 or "crust," commonly regarded as rigid but really somewhat 

 flexible, elastic, and plastic, may be considered as due, firstly, to 

 the gradual separation of the specifically lighter portions of the 

 molten mass under the influence of gravity, and secondly, to the 

 cooling and consequent solidification of those portions.* 



Now, since by the hypothesis, the cooling is constantly in 

 progress, and cooling results in a lessened volume, the solidified 

 crust, at first formed while floating on the molten mass beneath, 

 would, after a time, be left without support other than its own 

 (imperfect) rigidity, and would begin to subside. The result 

 would necessarily be a series of contortions, for, as Mr. Wilson 

 has clearly shown, "contortions are the inevitable result of the 

 subsidence of a curved surface."! Furthermore, general subsi- 



*It may be remarked, as a general rule, that rock-substances of low are 

 mostly less fusible than those of high specific gravity. 



•f-See J. M. Wilson, Geol. Mag., 18S5, p. 206. Mr. Wilson has mathematically 

 investigated this subject, and shewn that the known rising and sinking of large 

 areas of the earth's surface is adequate to produce such faultings and contortions 

 as are observed. 



