the same spot, compared with the average rate of subsidence, we may take the case of Tahiti, an island of 7,000 feet high. Now here the present barrier-reefs would never be continued upwards into an atoll, although, should the subsidence continue at a period long after the death of the present reefs, new ones might be formed high up round its sides and ultimately over it. The case resolves itself into: what is the ordinary height of groups of islands, of the size of existing groups of atolls (excepting as many of the highest islands as there now ordinarily occur encircling barrier-reefs in the existing groups of atolls)? and likewise what is the height of the single scattered islands standing between such groups of islands? Subsidence sufficient to bury all these islands (with the exception of as many of the highest as there are encircled islands in the present groups of atolls) my theory absolutely requires, but no more. To say what amount of subsidence would be required for this end, one ought to know the height of all existing islands, both single ones and those in groups, on the face of the globe--and, indeed, of half a dozen worlds like ours. The reefs may be of much greater [thickness] than that just sufficient on an average to bury groups of islands; and the probability of the thickness being greater seems to resolve itself into the average rate of subsidence allowing upward growth, and average duration of reefs on the same spot. Who will say what this rate and what this duration is? but till both are known, we cannot, I think, tell whether we ought to look for upraised coral formations (putting on one side denudation) above the unknown limit, say between 3,000 and 5,000 feet, necessary to submerge groups of common islands. How wretchedly involved do these speculations become. LETTER 534. TO E. VON MOJSISOVICS. Down, January 29th, 1879. I thank you cordially for the continuation of your fine work on the Tyrolese Dolomites (534/1. "Dolomitriffe Sudtirols und Venetiens": Wien, 1878.), with its striking engravings and the maps, which are quite wonderful from the amount of labour which they exhibit, and its extreme difficulty. I well remember more than forty years ago examining a section of Silurian limestone containing many corals, and thinking to myself that it would be for ever impossible to discover whether the ancient corals had formed atolls or barrier reefs; so you may well believe that your work will interest me greatly as soon as I can find time to read it. I am much obliged for your photograph, and from its appearance rejoice to see that much more good work may be expected from you. I enclose my own photograph, in case you should like to possess a copy. LETTER 535. TO A. AGASSIZ. (535/1. Part of this letter is published in "Life and Letters," III., pages 183, 184.) Down, May 5th, 1881.