My dear Darwin,—

There is no subject to which Judd oftener referred, and told me he had done so when discussing volcanic questions with Scrope, than your subsidence of St. Jago, as being a general law of volcanic regions.¹ The sinking down referred to by me (‘Principles’ vol. ii. 458, ‘Student’s Elements,’ 149–166), as occurring in New Zealand, and which has been confirmed by the New Zealand geologists, corroborates your St. Jago experience.²

Just before I left Scotland Judd made an excursion to revisit the Hebrides, and during a ten days’ absence discovered for the first time carboniferous strata preserved under some two thousand feet of tertiary basalt in the Island of Mull, and he brought to me what I guessed was a Sigillaria and the cast of a Calamite; but when I sent them to Charles Bunbury he pronounced them to be Lepidodendron aculeatum and a Calamite, ordinary carboniferous fossils.³ All this is explained by admitting that in the time of the Miocene volcanos there was a sinking down like that of St. Jago in the island, near the loftiest cone of that island, and one of the five great volcanos which he believes to have existed in the Hebrides, and which may have rivalled the Peak of Teneriffe in height, in the Miocene period. How much grandeur the scenery of the Hebrides must have presented in that same Miocene period, when Madeira was already in continued action, as well as Porto Santo and the Giant’s Causeway, &c.!⁴ The Duke of Argyll has sent me word that he has found a fossil Salisburia (now a genus growing in Japan) among the fossils of Ardtun in Mull, where he formerly found Asa platanoides.⁵ I wonder whether the cones of the Hebrides, if they were as high as Etna or the Peak of Teneriffe⁶ before the sinking down which you observed in St. Jago had taken place, were covered with snow in the Miocene period, when the vegetation at their base, at Ardtun for example, was sub-tropical. As there is now a vast difference in the vegetation of the desert region of Etna usually covered with snow, so in Miocene time it does not follow that volcanos ten or twelve thousand feet high should not have had on their summits a flora very different from that at their base; but these you will say are idle speculations.

It is remarkable how perfectly a sinking in Miocene times, like that which you have supposed or proved for the Cape de Verde Islands, would in Judd’s opinion give a satisfactory solution to the preservation in Mull and in the cliffs on both sides of the Sound of Mull, of those intercalations of poikilitic, triassic, liassic, oolitic, neocomian, cretaceous, and newer strata, of which a full account is to be given before the close of Judd’s paper.⁷

All the work which I have done with Judd in Forfarshire has confirmed me in the belief that the only difference between Paleozoic and recent volcanic rocks is no more than we must allow for, by the enormous time to which the products of the oldest volcanos have been subjected to chemical changes such as those which turn an olivine basalt into serpentine.

Ever affectionately yours, | Charles Lyell.