TUE EOYITIAN PETRI FIED FORESTS. 821 But whatever be the chemicul forces thnt have converted the nummulites into flints, these do not remain intact after their transformation. The vicissitudes of the temperature, which beneath these cloudless skies varies so greatly between day and night, cause the stones to chip, strewing vast spaces with their fragments. Occasionally the breakage of these flints is effected in such a way as to give them a perfectly regular fonn. Thus in a wudy of the Arabian range west of Beni-Sucf are met, scattered about in considerable quantities, siliceous fragments resembling truncated cones and presenting eight symmetrical facets. To the sudden changes of temperature have also been attributed the broken and even-worked flukes that have been found in various prehistoric workshops through- out Egypt. But human labour is so clearly stamped on these specimens that it is quite impossible to confound them with the products of natural causes. In the Libyan desert Zittel sought in vain for any naturally produced siliceous chippings bearing even a remote resemblance to the spear and dart heads worked by the men of the stone age, whether in Egypt, Europe or the New World. Amongst the stones of regular form found in the Egj'ptian deserts, Cailliaud and Russegger were the first to cull attention to the carnelians, jaspers, agates, and other hard stones pre- senting the form of lentils or discs of various dimensions, encircled by a round ridge somewhat like a ring. The interior of these natural specimens is often disjxjsed in concentric circles, and such concretions are very frequently found associated with fossil wood. The Egyptian Petrified Forests. By a remarkable contrast, petrified trees are known to occur in many parts of a region where living plants have become so very rare. On the east slope of the Jebel-Mokattam, not far from Cairo, is found, if not a " petrified forest," as it is usually called, or " masts of shipwrecked vessels," pierced with holes by phollades, and covered with marine deposits, as the curly truvellers pretended, at least a number of stems transformed to blocks of flint or chalcedony. But by penetrating farther into the desert we come upon far more extensive petrifications, which might really deserve the name of " forests." In a depression of the Arabian plateau, to the south- east of Cairo, the trunks of trees of all sizes are found in such multitudes that certain tracts are exclusively covered with the siliceous stems or fragments of fossil wood. In the Libyan desert, west of the Pyramids, other " petrified forests " contain stems over sixty feet long, with their roots and branches, and even with the bark still perceptible in some places. Travellers have also discovered similar masses of fossil wood in various parts of the Nubian desert, in Senar and Kordof&n, and even on the upland pluteaux of Abyssinia. In all these regions the vegetable remains BO petrified belong to the order of the sterculiaceoc. In Egj'pt the prevailing variety is the uichoUa Nilotica, and a species of bamboo obtained from these forests is also preser>'ed in one of the natural history collections in Cairo. Whence come all these stems of petrified trees ? Some geologists have suggested that they may have been washed up by the sea at a time when the Mediterranean 21— AP.
