ANNIVERSARY ADDRESS OF THE PRESIDENT. CXV 



on the generation of the minerals they contain, on the history of 

 their emission, which I have simplified, and on their natural clas- 

 sification." 



According to M. Durocher's view, silicium performs the same office 

 in the mineral kingdom as carbon performs in the organic world, 

 acting as a polybasic acid, and uniting with the oxides in very differ- 

 ent proportions, so as to give rise to a great variety of combinations ; 

 and hence that the minerals which constitute rocks must depend 

 principally upon the materials brought into chemical action. After 

 comparing his own chemical analyses with those recorded in the 

 works of other writers, M. Durocher concludes that the granites, 

 eurites or felspathic and quartziferous porphyries, trachytes, phono- 

 lites, pearlstones, obsidians, pumice, and lavas, rich in vitreous fel- 

 spar, were produced from the first of his magmas, whilst diorites, 

 ophites, melaphyres, euphotides, hyperites, traps, basalts, and augitic 

 lavas proceeded from the second. 



M. Durocher also states that, if the different varieties of one type 

 of rock, such as granite, be examined, there will be often found a 

 greater difference in their elementary composition than in that of 

 different species proceeding from the same magma ; as, for example, 

 those between granite and a trachyte or a pumice, and he then 

 arranges in a tabular form the several species supposed to proceed 

 from each magma ; remarking that the differences are equally due 

 to the conditions of pressure, temperature, and cooling as to mere 

 elementary composition, and he adds that the magmas which have 

 produced igneous rocks are similar to metallic baths containing 

 in a state of fusion various metals, which on cooling give rise to alloys 

 differing from each other according to the circumstances of solidi- 

 fication. He adds that, where the two layers or magmas are in con- 

 tact, hybrid products are formed, or rocks which have petrographic 

 and geological affinities, sometimes approximating them to the time 

 rocks of one magma, sometimes to those of the other. The upper 

 zone, rich in silica, and poor in earthy bases and in oxide of iron, 

 has the least density, and its specific gravity differs from that of the 

 lower as much as oil in this respect differs from water, — a circum- 

 stance to which is due the permanent separation of the two magmas. 

 The solid crust, therefore, of the globe rests, in M. Durocher's opinion, 

 upon a fluid zone composed of two distinct layers : the upi:>er, which 

 is the most refractory, being only semifluid or pasty ; the second much 

 more fluid and dense. It is to this second magma, so rich in oxide of 

 iron, that he ascribes the eruptions of oxidulous iron which have ap- 

 peared in the manner of igneous rocks in Italy, the Oural Mountains, 

 and Scandinavia, being connected with hornblendic or augitic rocks ; 

 and in the upper bed he considers that light or volatile bodies would 

 be concentrated, such as alkaline metals, fluor, boron, &c. : it is indeed 

 amongst granite rocks proceeding from this layer that the fluor-sili- 

 cates, boro-silicates, as mica, topaz, tourmaline, &c, have been usu- 

 ally found. There is of course much speculation in this theory of M. 

 Durocher ; but the attempt to penetrate through the mystery which 

 hangs over the production of the great variety of igneous rocks, and 



