GEOGNOSTIC PERIODS. 289 



are merely the ramifications of a lower deposit. The foci of 

 active volcanoes are situated at enormous depths, and judging 

 from the remarkable fragments which I have found in various 

 parts of the earth incrusted in lava currents, I should deem it 

 more than probable that a primordial granite rock forms the 

 substratum of the whole stratified edifice of fossil remains.* 

 Basalt containing olivine first shows itself in the period of 

 the chalk, trachyte still later, whilst eruptions of granite 

 belong, as we learn from the products of their metamorphic 

 action, to the epoch of the oldest sedimentary strata of the 

 transition formation. Where knowledge cannot be attained 

 from immediate perceptive evidence, we may be allowed from 

 induction, no less than from a careful comparison of facts, to 

 hazard a conjecture by which granite would be restored to a 

 portion of its contested right and title to be considered as 

 a primordial rock. 



The recent progress of geognosy, that is to say, the more 

 extended knowledge of the geognostic epochs characterised 

 by difference of mineral formations, by the peculiarities and 

 succession of the organisms contained within them, and by 

 the position of the strata, whether uplifted or inclined hori- 

 zontally, leads us, by means of the causal connection existing 

 among all natural phenomena, to the distribution of solids and 

 fluids into the continents and seas, which constitute the upper 

 crust of our planet. We here touch upon a point of contact 

 between geological and geographical geognosy, which would 

 constitute the complete history of the form and extent of 

 continents. The limitation of the solid by the fluid parts of 

 the earth's surface, and their mutual relations of area have 

 varied very considerably in the long series of geognostic epochs. 

 They were very different, for instance, when carboniferous 

 strata were horizontally deposited on the inclined beds of the 

 mountain limestone and old red sandstone ; when lias and oolite 

 lay on a substratum of keuper and muschelkalk, and the chalk 

 rested on the slopes of green sandstone and jura limestone. 

 If, with Elie de Beaumont, we term the waters in which the 

 jura limestone and chalk formed a soft deposit, the Jurassic or 

 oolitic, and the cretaceous seas, the outlines of these formations 

 will indicate, for the two corresponding epochs, the boundaries 



* See Elie de Beaumont, Descr. geol. de la France, t. i. p. 65; 

 Beudant, Geologic, 1844, p. 209. 



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