CX1V PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



constitution of igneous rocks for more than twelve years. Kesearches 

 of this kind illustrate the manner in which the simple elements of 

 the crust of the earth have been so combined as to produce rocks of 

 considerable apparent, but little real difference. It is for this pur- 

 pose that M. Durocher investigates with great care the atomic pro- 

 perties of the oxygen of the silica and of the bases of these magmas, 

 the solidification of which has produced the igneous rocks. In the 

 hornblendic series, the ratio of the oxygen of the silica to that of the 

 bases exceeds 3 to 1, except in trachytic lavas and phonolites ; so that, 

 on the magma solidifying, it is resolved entirely into a crystalline 

 mass, and the silica which is in excess becomes free in the form of 

 quartz. In granite, the proportion between the oxygen of the alu- 

 mina and that of the alkaline and alkali-earthy bases is, on the 

 average, 3*7 to 1 ; and, as there is more alumina than is necessary to 

 produce merely a felspathic mineral, the excess of alumina contributes 

 to the production of mica. In normal granite there is about 35 per 

 cent, of quartz, and 40 to 45 of felspar which has absorbed |-ths of the 

 alumina, the remaining J-ths serving to produce the mica in the pro- 

 portion of 20 to 35 per cent. ; and M. Durocher observes that the 

 same magma, on solidifying, may, through a variation of circumstances, 

 take the form of a granite, sometimes more rich in felspar, and some- 

 times in mica and quartz. Where the oxygen of the alumina in the 

 erupted mass was nearly three times that of the protoxides, very 

 little mica was found, and a pegmatite more or less rich in felspar 

 was the result. It is easy indeed to imagine how readily the varia- 

 tions of physical circumstances sometimes consequent on tranquillity, 

 sometimes on disturbance, must tend to alter the distribution of 

 the mineral elements, and thus to produce, on solidification, different 

 results ; it is curious also to observe how in the long series of 

 events the alumina becomes proportionally diminished, and gives 

 rise to newer forms of igneous rocks. M. Durocher states that, in 

 the felspathic rocks of the tertiary and recent epochs, the oxygen 

 of the alumina bears a proportion to that of the alkaline and 

 earthy-alkaline bases of less than 3 to 1 ; so that all the magma can- 

 not be changed into felspar, and part continues in the state of a 

 paste, or forms other minerals less aluminous than felspar. These 

 and many other variations in the resulting rocks produced by the 

 solidification of the magma are described by M. Durocher, and the 

 general atomic relations shown by tables ; and this is the interesting 

 conclusion of one of his papers, " the present essay has exhibited 

 the physical, chemical, and geogenic relations which connect together 

 the igneous rocks, however varied in their aspect. The clearness with 

 which these relations have been unravelled confirms the proposition 

 I have laid down, that all igneous rocks have been derived from two 

 mineral beds or layers, situated below the crust of the earth — one 

 characterized by its richness in silica, and the other, though poorer 

 in silica and in alkalies, containing a very much larger proportion 

 of alkaline earths and oxide of iron, and at the same time dis- 

 tinguished by very different atomic proportions. I have thus suc- 

 cessively thrown light upon the mutual relations of eruptive products, 



