THE EVOLUTION (W PETKOLOGICAL IDEAS. 301 



1 can not leave this portion of the suliject without calling" attention 

 to the recent work of Professor Joly on the melting- points of the 

 rt)ck-forniino- minerals, anclhis proof of the enormous rang-e of viscosity 

 possessed by quart/ and other minerals. 



Whatever view we take as to the nature of silicate magmas, there 

 can be no doubt that in general the process of consolidation is a process 

 of dijftcrentiation. Detinite compounds separate out, either succes- 

 sively or simultaneously, from a homogeneous magma, and at the time 

 of their formation are in equilibrium with the surrounding liquid; 

 but owing to changes in temperature and pressure the equilibrium 

 established at one period may ho destroj^ed at another, and the igneous 

 rock as we see it may not contain a record of all the operations which 

 have taken place during the process of consolidation. So far as indi- 

 vidual rocks are concerned, we look to experiment rather than to 

 observation to give precision and deliniteness to our ideas regarding 

 the nature of the changes which accompany solidification. 



THE ORIGIN OF SPECIES. 



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

 individuals, l)ut 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 Professor Iddings and Professor Lcewinson-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 trach^'te and ))asalt. Darwin (1844). in his impor- 

 tant work on Volcanic Islands, also discussed the origin of petrograph- 

 ical species. He directed attention to two causes of ditferentiation 

 which may ultimately prove to be of great importance — (1) the move- 

 ment of crystals in a magma under the intluence 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 lifjuation (but quite distinct from the process referred 

 to by Durochei- under the same name), by means of which silver is 

 separated from blister copper. The copper is fused with a cei-tain 

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

 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 



