Vol. 57.] ANNIVERSAEY ADDRESS OP THE PRESIDENT. Ixxi 



directed towards a particular end. The constituents of massive 

 rocks are divided by Prof. Eosenbusch into four groups : — 



1. The ores and accessory constituents (magnetite, hcematite, ilmenite, 



apatite, zircon, spinel, and titanite). 



2. The ferromagnesian constituents (biotite, hornblende, pyroxene, and 



olivine). 



3. The felspathic constituents (felspar, nepheline, leucite, mehhte, sodalite, 



and haliyne). 



4. Free silica. 



Prof. Rosenbusch pointed out that members of the first group 

 precede those of the other groups ; that in granites and syenites 

 the members of the second group precede those of the third ; but 

 that in the diabases and gabbros the order is inverted, and that in 

 both groups silica is the last. The general conclusion is reached 

 that 



' the order of consolidation of the silicates and, consequently, their crystallo- 

 graphic development (idiomorphism), corresponds to a law of decreasing 

 basicity ; the ores and accessory minerals are the earliest, and quartz is tlie 

 latest, product of the rock- forming process.' 



This empirical law expresses, in a broad and general way, the 

 main facts observed with regard to the sequence of minerals in 

 thelarge and important group of intermediate rocks, but it breaks 

 down when applied to the most acid and the most basic rocks ; 

 quartz is often formed before felspar in the former, and iron-ores 

 are not infrequently formed after felspar in the latter. 



The views that we hold regarding the laws which express the order 

 of consolidation in igneous magmas will necessarily be coloured by 

 our conceptions as to the nature of these magmas. A great advance 

 in the evolution of ideas on this subject is marked by a short letter, 

 written by Bunsen to Strong, and published in the Journal of the 

 German Geological Society for 1861. In this letter Eunsen points 

 out that the arguments against the igneous origin of granite, so far 

 as they rest upon the so-called anomalous order of consolidation 

 of the minerals, are based on a misconception of the nature of the 

 process of consolidation. He says : — 



' The temperature at which a substance consolidates from a state of fusion 

 is never that at which it separates from a solution in another substance. 

 The temperature at which a definite substance crystallizes from its own 

 liquid depends only on the substance and on the pressure to which it is 

 subjected ; whereas the temperature at which the same substance separates 

 from its solution in another substance depends principally on the relative 

 proportions of the two substances. No chemist will fall into the ei'ror 



