My dear Sir

Although your Excellency must be overburthened with business, I cannot resist the temptation to thank you cordially for the very kind, & if I may be permitted to say so, admirable spirit, with which you excuse & tell me to forget, the to me painful, origin of our correspondence.¹ I have been the more gratified by your letter, as I had not the least expectation of hearing from you.—

I am extremely glad to know how well your Colony is now prospering.² Ever since the voyage of the Beagle, I have felt the deepest interest with respect to all our colonies in the southern hemisphere. However much trouble & anxiety you must have had & will still have, it must ever be the highest gratification to you to reflect on the prominent part you have played in two countries,³ destined in future centuries to be great fields of civilization.—

You are so kind as to offer aid in any Natural History researches in New Zealand: I have no personal interest on any point there; but there are two subjects which have long appeared to me well deserving investigation; & if hereafter your labours should be lightened you might like to attend to them yourself, or direct the attention to them of any Naturalist under you.— The first is, an examination of any limestone caverns: such exist near the Bay of Islands⁴ & I daresay elsewhere: I was prevented entering them by their having been used as places of Burial. Digging in the mud under the usual stalagmitic crust, would probably reveal bones of the cotemporaries of the Dinornis. I think there is a special interest on this point, from New Zealand being at present so eminently instructive in a negative point of view, with respect to the distribution of terrestrial mammifers.⁵ I have long ardently wished to hear of the exploration of the New Zealand Caverns.

The second point is, whether there are “erratic boulders” in New Zealand, more especially in the middle & southern islands, & their northern limit, if such occur. Most geologists are now united to considering erratic boulders, to have been transported by icebergs or glaciers. I consider it as a most important question, as bearing upon the former climate of the world, to know whether such proofs occur generally in the S. hemisphere as in the Northern: I have ascertained that such is the case in S. America, from Cape Horn to about Lat 40^o. This subject requires much care & some little knowledge or at least thought. I saw inland of the Bay of Islands, large rounded blocks of greenstone, but I was unable to ascertain whether the parent rock was far distant; nor did I then see the full importance of the question, otherwise I would have devoted every hour to it. Very large, angular blocks of foreign rock, lying on isolated hills or hillocks offer the best & without much care, the only sure evidence. We do not know the extent to which during ages, the waves of the sea, at various alternating levels, with earthquake waves &c & occasional heavy floods, may transport in valleys & over an undulating surface very large boulders, hence becoming rounded. Granite from its tendency to orbicular disintegration has given rise to several false accounts of erratic boulders.—

In the North, & in S. America, erratic boulders are accompanied by thick masses of mud, (called in Scotland till), which are characterized by containing angular & rounded stones of various sizes, arranged in utter confusion, & without stratification. The till is believed to have been produced by the incessant grounding of charged icebergs. The age of the erratic phenomenon would be a highly important element, & can sometimes be discovered by shells in the till, or by shell-beds under or above the boulders. In N. America & probably S. America the great extinct quadrupeds existed after the ice period: Do boulders lie upon the Dinornis alluvial beds.—

If I have wearied you with these details, I beg to apologise & you can burn this letter; but I thought, perhaps, you would not object to hear my opinion on two, as I believe, really curious subjects for investigation. I would, myself, go through much labour to investigate the erratic phenomena & trace its limits & age. Should you ever obtain any evidence on this head, it would delight me to hear the result.

Again allow me to thank you very cordially & I beg your Excellency to believe me | Your sincerely obliged | C. Darwin To His Excellency | Sir G Grey⁶