35. Another species of fossil salt exhibits appearances equally favourable to the theory of igneous consolidation. This is the Trona of Africa, which is no other than soda, or mineral alkali, in a particular state. The specimen of this fossil in Dr Black's, now Dr Hope's, collection, is of a sparry and radiated structure, and is evidently part of the contents of a vein, having a stony crust adhering to it, on one side, with its own sparry structure complete, on the opposite. It contains but about one sixth of the water of crystallization essential to this salt when obtained in the humid way; and, what is particularly to be remarked, it does not lose this water, nor become covered with a powder, like the common alkali, by simple exposure to the air. It is evident, therefore, that this fossil does not originate from mere precipitation; and when we add, that in its sparry structure it contains evident marks of having once been fluid, we have little reason to entertain much doubt concerning the principle of its consolidation. Thus, then, the testimony given to the operation of fire, or heat, as the consolidating power of the mineral kingdom, is not confined to a few fossils, but is general over all the strata. How far the unstratified fossils agree in supporting the same conclusion, will be afterwards examined. 3. _Position of the Strata._[12] [Footnote 12: Theory of the Earth, vol. i. p. 120.] 36. We have seen of what materials the strata are composed, and by what power they have been consolidated; we are next to inquire, from what cause it proceeds, that they are now so far removed from the region which they originally occupied, and wherefore, from being all covered by the ocean, they are at present raised in many places fifteen thousand feet above its surface. Whether this great change of relative place can be best accounted for by the depression of the sea, or the elevation of the strata themselves, remains to be considered. Of these two suppositions, the former, at first sight, seems undoubtedly the most probable, and we feel less reluctance to suppose, that a fluid, so unstable as the ocean, has undergone the great revolution here referred to, than that the solid foundations of the land have moved a single fathom from their place. This, however, is a mere illusion. Such a depression of the level of the sea as is here supposed, could not happen without a change proportionally great in the solid part of the globe; and, though admitted as true, will be found very inadequate to explain the present condition of the strata.