REPORT ON THE PETROLOGY OF THE ROCKS OF ST. PAUL. 27 



On the supposition that the rock belongs to the schists, we must necessarily 

 suppose the existence of an upheaval of the earth's crust. The beds, of more or less 

 considerable thickness, which formed the entire mass in which the peridotite was 

 encased, obeying a heaving motion (the effects of which are so conspicuous in analogous 

 rocks), must have risen above the water, and then, being attacked by the erosive action 

 of the waves, the outer portions which covered the peridotite were disintegrated and 

 removed, leaving behind them as a fragment of the primitive mass nothing but what 

 we now see of the Rocks of St. Paul. 



It is important not to lose sight of a fact already referred to concerning the resistance 

 offered to erosive actions by a rock so massive and compact as this peridotite. Though 

 peridotites have often undergone transformation into serpentine, it does not follow that 

 they are so easily alterable as is often stated, or that the resistance they offer to mechanical 

 force is not stronger than that which they oppose to hydro-chemical action. It may well 

 be supposed that, at the point now occupied by these rocks, there formerly rose a mass of 

 ancient rocks, the original dimensions of which may have been successively reduced by 

 mechanical and chemical phenomena. Such an interpretation of the history of the locality 

 is neither contrary to the nature of the rock, nor to the details, still very incomplete, 

 which we have on its geological structure and relation, for among the schisto-crystalline 

 formations on the surface of the continents many an example might be cited of a similar 

 series of geological changes. 



It is scarcely necessary to add that the opinion which tends to see in the Rocks of 

 St. Paul an outcrop of the ancient strata is not antagonistic to that which assigns to the 

 oceanic basins a constancy in the general disposition, maintained during long geological 

 ages. As regards the possibility of the existence of a continental mass in the Atlantic at 

 periods not very remote from our own, with which the island of St. Paul might be 

 supposed to have been connected, it must be confessed that soundings have shown no 

 traces of it, and that the island of St. Paul affords no proof of subsidence. There 

 are no sedimentary formations, either fresh-water or marine, to point to a greater 

 extent of land-surface in former geological ages, such as is noticed in some other 

 Atlantic islands. 



The absence of fauna and flora on these rocks relieves our problem of the biological 

 difficulties which other less barren oceanic islands present. 



It is well known that Lyell, 1 in opposing the opinion held by many naturalists, and in 

 particular by Edward Forbes, that the Azores, Madeiras, and Canaries are the last 

 remaining fragments of a once continuous area of land that connected them with the West 

 of Europe and North Africa, laid great stress on the enormous depressions existing between 

 these regions, because such a theory " involves an amount of change of level so vast, that 

 to assume its occurrence since the close of the Miocene epoch, is quite inconsistent with 



1 Lyell, Principles of Geology, vol. ii. p. 410. 



