My dear Lyell

I cannot say what are the southern or northern limits of true coral islands in Pacific, for scacely any islands occur south or north of the Tropic.— Bermuda in Lat 33^o 1/2. is an exception to general rule owing to Gulf Stream— Coral is abundant at Mauritius on Tropic & I believe at Loo Choo¹ a little north of T of Cancer (that is 23^o 30’) It will be difficult for you to talk of great areas abounding with corals.— People’s ideas of the Pacific are most false. — In the thick archipelagoes—in a long days sail, you will often only see one or two islands. The Dangerous Archipelago² is (speaking very roughly & accuracy cannot be obtained in such a point) is an oblong of 1200 miles in length, by 300 or 400 in breadth, which is thickly strewed with lagoon Isl^d. But mind I use the word thickly, as before cautioned, that is with reference to the immense open ocean of the Pacific.— Again The Radack³ & Ralix⁴ islands, together with the Kingsmill islands, may be similarly included by 1100×200 or 300 miles.— The Maladive & Laccadive will offer you another example.— A NW by W linear area of a length of between 4000 & 5000 (& of breadth of from 200 to 400 miles) including the Dangerous Arch: & the Radack Arch., the Letilia group & many other scattered islands, I believe, includes nothing but lagoon island.— But then there are spaces of some hundred miles without an island of any kind.—

The sea proposed by Flinders⁵ to be called the Corallian Sea, is bounded on East & West by Australian Barrier, & the great reefs of New Caledonia, & to the N. by the reefs of Louisiade.— This area thus surrounded on 3 sides by coral & containing a few reefs is about,— the dimension here given.—

But then the land to which these reefs are attached is primary.—⁶ I fear this information will scarcely be of any service to you; but it is the best I can give.—⁷ Many thanks for the books.— I am afraid I shall not be able to read them for nearly a week, as I have not finished E. de B.—⁸

Yours very truly, C Darwin—

Tuesday Night.

With respect to your E & W. Boundary of coral in the Pacific, it is scarcely possible to give any satisfactory answer for the two great areas, the Dangerous or Low Archipelago, and the Corallian sea are separated by great volcanic band, in which coral exists in very small quantity, or is entirely absent.—