ANlSriYEESARY ADDEESS OP THE PRESIDENT. Ixxxiii 



flint. After describing the changes which dead corals undergo, 

 and giving a full account of the different forms of mineralization 

 which result therefrom, whether calcareous or siliceous, or one or 

 more combinations of the two, Dr. Duncan states that he does 

 not believe in the hypothesis which attribu.tes the silicification of 

 the West Indian fossil corals to a volcanic outburst which poured 

 siliceous solutions over the depressed reefs of the Miocene age. 

 He shows that this silicification of fossils has been a slow process, 

 having no other origin than in those chemical operations Avhich 

 are still going on, and that their greater or less intensity in 

 certain favourable localities has produced siliceous fossils, even 

 amongst those affected by the calcareous form of mineralization. 

 A kind of chemical transposition takes place under certain cir- 

 cumstances. Silica, it is observed, has an affinity for bodies 

 formerly organized, and often destroys the former tissues. There 

 is a small amount of silica in corals ; and in microscopic sections 

 of Antiguan corals, in which silicification is incomplete, the silica 

 is usually seen deposited in molecules in the centre of the calca- 

 reous mass, and not on its superficies. The accumulation of the 

 silica is no doubt determined by the persistence of some animal 

 or vegetable organic tissue decomposing more or less slowly, 

 and by which it is attracted, until at last the whole structural 

 details of the coral are destroyed by the deposition of homoge- 

 neous black flint. And finally, he observes that every grade of 

 a silicification which destroys the textures of corals, and reduces 

 them at last to pure homogeneous black flint, resembling that of 

 the Upper Chalk, may be distinguished. 



M. Charles Martins of Montpellier, recently elected a Poreign 

 Correspondent of this Society, has published, in a recent Number 

 of the ' Eevue des deus Mondes ' (July 15, 1864), an interesting 

 account of the physical characters of the great Sahara or desert 

 in the province of Constantino. It contains some interesting 

 details of the physical geography of the country, bearing on those 

 geological problems which we are constantly discussing. 



Describing the route from El-Kantara to Djebel-el-Mela, with 

 its beds of rock-salt, he says, " We then entered a district com- 

 posed of gi'ey, blue, yellow, and red marls, associated with conglo- 

 merates and limestones, cut up into deep ravines by the torrents 

 which, during the rainy season, descend from the rock-salt moun- 

 tains. These ravines, from 50 to 60 metres in depth, were so 

 close to each other, that it would have required several days to 

 reach the foot of the mountain, distant only a few kilometres in 

 a straight line, through this labyrinth of gorges separated by sharp 

 narrow ridges. Let those geologists who wish to describe the 

 erosive action of pluvial waters, set aside the wretched examples 

 they quote to illustrate their argument ; let them visit Algeria, and 

 gain their inspirations from the ravined district of Djebel-el-Mela 

 and the mountains of the Kabyle. There they will see how the 

 erosive power of water is able, under our very eyes, to transform 

 a level plain into a mass of mountains as varied and broken in their 



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