Ixxviii PEOCEEDIX&S OF THE GEOLOGICAL SOCIETY. [May IQOT, 



The Oeigin" of Species. 



The geologist, however, has to deal not only with igneous rocks 

 as individuals but as groups, to consider their mutual relations, their 

 geographical distribution and mode of origin. But to give anything 

 like a full account of the growth of ideas on this subject would 

 expand this address to an inordinate length, and would, moreover, 

 be a work of supererogation, for the whole question has been 

 admirably reviewed by Prof. Iddings and Prof. Loewinson-Lessing. 



The germs of all the theories which are now struggling for 

 existence can be discovered in the writings of our predecessors. 

 Scrope (1825) held the view that lavas were formed from previously 

 crystallized rocks, such as granite, and maintained that in the 

 process of eruption, or intumescence as he termed it, a kind of 

 differentiation might take place, giving rise to trachyte and basalt. 

 Darwin (1844), in his important work on Volcanic Islands, also 

 discussed the origin of petrographical species. He directed attention 

 to two causes of differentiation which may ultimately prove to be of 

 great importance — (1) the movement of crystals in a magma under 

 the influence of gravity ; and (2) the squeezing or leaching-out of 

 the more fusible constituents from a partially consolidated or 

 partially fused mass. The first of these he illustrated by the well- 

 known Pattinson process for desilverizing lead, and the second 

 might be illustrated by another metallurgical process often known 

 as liquation (but quite distinct from the process referred to 

 by Durocher under the same name), by means of which silver is 

 separated from blister-copper. The copper is fused with a certain 

 proportion of lead, and the bars are maintained at a temperature 

 above the fusing-point of the silver-lead alloy and below that of 

 copper. The silver-lead alloy is thus leached out of the copper, 

 which remains as a solid porous mass. Such a separation might be 

 effected in the case of a plutonic mass, if a partially solidified 

 magma were subjected to pressure under conditions which admitted 

 of the escape of the still liquid portions into the surrounding rocks. 

 As a matter of fact it has been so applied by Mr. Barrow, who 

 thus explains the relation between pegmatites and certain oligoclase- 

 biotite-gneisses in the Southern Highlands of Scotland. The eurite- 

 veins in granite are generally supposed to owe their origin to a 

 somewhat similar action, but in this case the separation is due to 

 the leaching-out of the still liquid eutectic into cracks in the nearly 

 consolidated mass, and not to orogenetic movements. It is com- 

 parable, therefore, to the liquation-process above mentioned. 



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