P a n ic um t u k g id u m , (Forsk. Arab. p. 18 ; Delile, Flore d’Egypte, p. 19, - tab. 19, f. 2.), is *lso one of the most common grasses from Tripoli to Bornou. Of A c o t y l e d o n e s , the only plant in the collection is Acrostichum velleum, found on the Tarhona mountains, Mr. Ritchie’s herbarium contains, also, a single plant of the same family, namely Grammltis Celerach. The foregoing observations have extended much beyond the limits which the number and importance of the plants they relate to may seem to require. 1 still regret, however, that I cannot add a few remarks on such species as, although not in the herbarium, were observed, either indigenous or cultivated, in the countries visited by the mission, and for information respecting which I am indebted to Major Denham and Captain Clapperton. But it being determined no longer to delay the publication of the very interesting Narrative, to which the observations already made will form an Appendix, I am unable at present to enter on this part of my subject. No. X X III. Letter to Major Denham, on the Rock Specimens bxougftfjfom Afçjjçft.,,, By Charles Konig, Esq. F.R.S. British Museum, Feb.25tlij 1Ô26. My deaf Sir, I have great pleasure in transmitting to you, for whateirèf iisé'ÿoü may think proper to makfe’ of it in the Appendix to yoiir forflicbrtimg Wbrk, the little I liaVe to advance on the geological and a few other dbjëèts,’' that were collected by yoin the late Dr. Ondney, and Captain Clappertori, oii yOtir journey through the'great African desert) and are nowdeposited in the British Museum. I t ch'iefly consists of the descriptive catalogue’ of a "small serife's of rock specimens, originally drawn up by me without particular fégafd tb’ théir geognostic occurrence ; to which I now prefix a few desultory remarks that occurred to me, when, with a view to its publication, I subjected that list to a second perusal. My materials are, indeed, very scanty ; but their description may, nevertheless, (in conjunction with the observations dispersed in the body of the work, of which I have not been able to avail myself) lead some of your Geological readers to more or less important results relative to the structure of the tract of country in which they were collected. There are among the specimens I have examined none that might be referred to the primitive formations, except those gathered south and west of Kouka. The principal specimen of granite (No. 1 .) brought by you from the Mandara mountains strongly resembles some of the fresh large-grained varieties of the same from the Fetish rocks in Congo. Those from Soudan, with feldspar, in its progress to kaolin, (No. 5.) betray the principal cause of the striking appearance of the granite mountains in that part of central Africa. The effects of atmospheric influence on that component, spreading chiefly in the direction of the natural rifts of the rock, are, in the Soudan mountains as well aS in those of the Hartz, the Riesengebirge, and other European granitic districts, manifested partly by the immense and numberless blocks, wholly or partially detached, and confusedly piled up on the sides of the mountains, or
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