My Dear Sir,

You will think me guilty of neglect in not answering your letter sooner, but I had gone for a short time to the Borders in search of picturesque beauty and poetic associations, and during the 3 days I remained in Edinburgh on my return I was much occupied in preparing a medical paper for the press,¹ and in consulting books. I put your letter in my pocket on leaving for a pedestrian tour across from Blair Gowrie to the head of the Dee and the Cairn Gorm, intending to answer it from Braemar—but on opening it for the purpose on a very rainy day when we could not venture on the mountains, I found I had taken your former letter by mistake—so I put off till I came home, and now I have been waiting for 10 days, in vain, for part of my baggage which has been taken somewhere else by the steam boat. This is enough of your time occupied by excuses—so I need not add, that the delay has enabled me to get some information that, I hope, will be of use to you, from a friend who was some years amongst the islands between the Isle of France and Africa.—

Before I got your last letter the objection to the Glen Roy gulf theory mentioned by yourself had occurred to me,² and I talked it over with Mr Geo. Anderson of Inverness (Author of the Guide)³ who knows more of the highlands than any body else. He said that he knew of no other similar appearance—but would not give me an opinion on your theory. I read M^c Cullochs paper while in Edinburgh, which bears every mark of a most acute and philosophic observer, and I willingly confess that much of his details can only be explained on your theory.⁴ I have not yet received your paper from London, and still hope to be able to visit the Glens before the Season is too far advanced.—

Now for Arabia.—⁵ Sahar is a considerable town on the coast of Hadhramaut in about Lat 14^o N. Long 49^o and about 40 miles E of Macula, on a coast exposed to a furious surf from the Indian ocean. Hot springs occur at Macula and at Hami, a village 14 miles E of Sahar. Behind the town an elevated indulating plain extends about 2 miles inland, and along the coast to some hills 3 or 4 miles on either side, which is covered with sand and well rounded pebbles of compact limestone containing fragments of shells. The sand is so saline, that after a shower I found the prints of the camel’s feet covered with it like ice, yet many aromatic plants and lillies made the desert to bloom like the rose, although at a distance the waste seemed devoid of any green thing. A little more than a mile N.E of the town, I found the imperfect remains of a fish in flinty slate, which dips to the South at a considerable angle, and rests on a soft clay slate and alternates with sandstone and conglomerate. on the edges of these strata a thin bed of consolidated gravel approaching to conglomerate rests horizontally and appears to belong to the same period as the gravel and sand. The pebbles, many of which are of considerable size, are all rounded.

A little east of this, the wide bed of a torrent has cut through the plain, and displays its structure in some low cliffs composed of horizontal strata of hard conglomerate consisting of limestone pebbles, madrepores, corals and shells cemented by a very calcareous sandstone. The corals and shells seemed to belong to species common in these seas.⁶ The cliffs were not above 16 feet in height from the fresh water that lay in pools in the bed of the stream, but the plain rose towards the town, near which are wells from 30 to 40 feet deep containing saline and bitter water. The water in a pool at the foot of the coral rock was sweet and being exposed to the air had a temperature of 70 (December), and that of the wells no less than 83$\frac{1}{2}$ ^o.— I went 6 miles into the interior to visit some very profuse hot springs that have formed a hill of Travertine, and in the way passed a gorge (caused by the torrent) through elevated ridges of schist and crystalline limestone containing shells, on the summit of one of which is an ancient fortress probably built by the Ethiopian princes whose monuments are scattered over Southern Arabia. This is 4 miles from the town, and a mile further are some rounded hills entirely composed of partially rounded stones, externally resembling basalt, but consisting of corals of different families much silicified. Their characters have not yet been examined—but I fancy that they may have been silicified by the agency of the neighbouring hot springs while at the bottom of the sea. We cannot say that these are recent. Mr Lonsdale was a good deal interested in them.

Aden consists of vast cliffs of basalt with olivine calcedony, &c, and on the summits pitch stone; and has probably been a vast crater with smaller ones in the centre, one side of which has been removed by the sea. Were it sunk below the deep, it would form a lagoon island, but I found no fossils above the influence of storms, which have thrown masses of recently consolidated sand and shells high on the shore. Here I first observed the connection of springs of pure water with the volcanic rocks and high temperature, while the saline springs were cold and derived from the superficial strata. The country around Mocha is Basaltic and has many sweet thermal springs—but the black soil near the town is very saline. Various schistose rocks occur at Hodeida, Ghezan &c and in some places the sea is encroaching. —but the island of Kamran north of Hodeida is composed of low cliffs, which rise 30 and 40 feet above a saline sandy plain containing sea shells, and not now covered by the sea at any time. It is here that the country ships enter the inner navigation where the water is quite smooth in consequence of a succession of reefs and islands.⁷ These cliffs are formed of a solid rock of corals and vast numbers of shells with a little calcareous cement. These appear to be all of recent species. In one place I found the coral rock resting on a ferruginous fine grained argillo-calcareous stone which I at first took to be a volcanic production, and still think that it has something of that nature. In a little valley between two of these cliffs there is a fine well of very pure water, and between it and the sea some fruitful date gardens, and on the cliff above some ruined tombs and an old castle. The coral rock supports only a few stunted grasses and aromatic herbs. Here then is an island that has been raised at least 40 feet above the highest level of the sea.⁸ You will see that it is not far from Gebel Tor,⁹ nor from the volcanic rocks on the mainland.— I landed on some other islands formed of coral and blown sand, but without any signs of elevation; and we often passed through narrow channels between coral reefs over which the sea broke with fury, and we frequently sailed in a narrow channel between them and the shore. (I was in an arab vessel). The harbour of Jedda itself is approached through these reefs, from which nothing can be learned as to the elevation of the coast, or as to the coral forming a mere coating: but the rising ground on which the town stands, and towards the interior near the road to Mecca where great cisterns for rain water are formed, and on a gentle eminence north of the town on which a great military Hosp^al. was being built, is entirely composed of a coarse calcareous sandstone abounding with Echini, large oysters, &c I believe of recent species. The specimens themselves are with Mr Sowerby, who thought the Kamran ones certainly recent, but before naming them