The configuration of intertropical South Africa had long been with him a subject of earnest study, and now he had come clearly to the conclusion that the middle part was a table-land, depressed, however, in the centre, and flanked by longitudinal ridges on the east and west; that originally the depressed centre had contained a vast accumulation of water, which had found ways of escape through fissures in the encircling fringe of mountains, the result of volcanic action or of earthquakes. The Victoria Falls presented the most remarkable of these fissures, and thus served to verify and complete his theory. The great lakes in the great heart of South Africa were the remains of the earlier accumulation before the fissures were formed. Lake 'Ngami, large though it was, was but a little fraction of the vast lake that had once spread itself over the south. This view of the structure of South Africa he now found, from a communication which reached him at Linyanti, had been anticipated by Sir Roderick Murchison, who in 1852 had propounded it to the Geographical Society. Livingstone was only amused at thus losing the credit of his discovery; he contented himself with a playful remark on his being "cut out" by Sir Roderick. But the coincidence of views was very remarkable, and it lay at the foundation of that brotherlike intimacy and friendship which ever marked his relation with Murchison. One important bearing of the geographical fact was this; it was evident that while the low districts were unhealthy, the longitudinal ridges by which they were fringed were salubrious. Another of its bearings was, that it would help them to find the course and perhaps the sources of the great rivers, and thus facilitate commercial and missionary operations. The discovery of the two healthy ridges, which made him so unwilling to die at the mouth of the Loangwa, gave him new hopes for missions and commerce. These and other matters connected with the state of the country formed the subject of regular communications to the Geographical Society. Between Loanda and Quilimane, six despatches were written at different points[45]. Formerly, as we have seen, he had written through a Fellow of the Society, his friend and former fellow-traveler, Captain, now Colonel Steele; but as the Colonel had been called on duty to the Crimea, he now addressed his letters to his countryman, Sir Roderick Murchison. Sir Roderick was charmed with the compliment, and was not slow to turn it to account, as appears from the following letter, the first of very many communications which he addressed to Livingstone: [Footnote 45: The dates were Pungo Andongo, 24th December, 1864; Cabango, 17th May, 1855; Linyanti, October 16, 1855; Chanyuni, 25th January, 1856; Tette, 4th March, 1856; Quilimane, 23d May, 1856.] "16 BELGRAVE SQUARE, _October 2_, 1855.