BY LEO A. COTTON. 511 



the galeiia-veins are so closely related topographically and geolo- 

 gically that a common origin must be assumed for them. The 

 same is true of the cassiterite veins and the various silver-lead- 

 ore-formations of the E7-z(jehirye; and the peculiar tin-bearing 

 silver-lead veins of Bolivia may be recalled in this connection." 



*' These intermediate groups warrant the conclusion that there 

 can have been no absolute essential difference between the genesis 

 of the cassiterite- and that of the silver-lead-veins. If the tin- 

 veins are to be explained by magmatic extraction, the silver-lead 

 veins may not be attributable to the work of underground water." 



Again, Spurr,* in considering the sequence of ore-deposition, 

 has summarised his views as follows : — 



" In an earlier paperf setting forth a theory of ore-deposition 

 it was proposed by the writer that the most important class o£ 

 ore-deposits (save of the most common metals) were differentia- 

 tion-products resulting from the siliceous extreme of the differen- 

 tiation of rock-magmas; that the successive steps of this metal - 

 depositing stage of the differentiation followed one another in a 

 normal regular order, and were deposited in successive zones, 

 according to temperature. It was also pointed out that, with the 

 downward progress of the cooling of the parent magma (and the 

 consequent sinking of the isogeotherms), the successive zones of 

 mineralization would migrate downward, and successive cemen- 

 tations of successive openings at a single horizon would show the 

 superposition of one zone upon another originally distinctly- 

 lower." 



" A preliminary attempt was also made to define some of the 

 principal zones as follows : — 



" 1. The pegmatite zone, containing tin, molybdenum, tungsten, 

 etc., with characteristic gangue minerals, such as tourmaline 

 topaz, muscovite, beryl, etc. 



* Spurr, J. E., " Ore-Deposition at Aspen, Colorado." Econ. Geol. Vol. 

 iv., No.4, June, 1909, p. 318. 



t Econ. Geol. Vol.ii., No.8, December, 1907, pp. 781-795. 



