ON ALTERED TERTIARY ROCKS NEAR CAIRO. 3 



to the right of the road from Cairo to Abus-abel, rising in a sharp broken 

 outhne to the height of 360 Paris feet above the Mediterranean. In 

 this hill there are some distinct traces of a volcanic eruption : even as 

 seen from a distance it is distinguished by its conical shape, and it is 

 also remarkable by the strong contrast of its brownish-red colour with 

 the pale yellow of the desert. Dschebel Achmar is wholly composed 

 of vitrified sandstone, and forms a distinct group of conical hills. In 

 the midst of these there is a wide crater-like hollow, the bottom of 

 which exhibits great inequalities, and which has openings on the N.W. 

 and S.E. The chief group, which encloses the crater, if it can be so 

 called, has a circumference of nearly 4000 fathoms {klafter) = 4^ 

 miles. In the bottom of this crater there are several holes, how deep 

 I know not, which are perhaps rents in the rocks, that played an im- 

 portant part in the eruption, if there ever was one. The walls of 

 these sloping cracks are completely vitrified ; and to suppose that to 

 have been produced or even helped by artificial means is out of the 

 question. One of these clefts is still open to a considerable depth, 

 and then ends in broken masses. From Dschebel Achmar many 

 similar conical hills may be seen, but none of them equal to it in ex- 

 tent. It really appears as if the whole ground under the strata of 

 coarse limestone of the Mokattam had been in a state of volcanic ac- 

 tivity ; that the molten mass had burst through in several places and 

 overflowed at the surface, causing new secondary fusions and meta- 

 morphoses of the rocks. For besides the vitrified sandstone on 

 Dschebel Achmar, we see several kinds of stone which have all the 

 appearance of being nothing else than the rocks of the Mokattam, 

 vitrified and half melted. We see there, in short, the sand of the 

 desert, melted and in the state of frit, as well as the dila^dal sandstone 

 of the isthmus. We see the sandy iron-shot clay that lies between 

 the siliceous limestone and the superior nummulite limestone, burnt 

 and melted. We see melted and vitrified siliceous limestone, vitrified 

 white earthy limestone, and also melted nummulite limestone, "\^ith 

 its included nodules and fossils wholly changed by the fire. We found 

 in the half-melted iron-shot clay some fossil wood, quite similar to that 

 commonly occurring in the Mokattam and its neighbourhood, but 

 entirely converted into hornstone ; also a white granular quartz in the 

 state of a frit, probably a melted lower bed of sandstone ; and lastly , 

 we found some basalt-like rocks, but without olivine, and consequently 

 their basaltic nature is certainly doubtful ; and these rocks appear to 

 rise from a considerable depth, for I never saw them resting on the 

 ground. 



*' If, taking into account w^hat has just been said, I consider the 

 appearances which the localities present, the finding all the rocks of 

 the Mokattam, but in an altered condition, and not, as it w^ould seem, 

 by the action of fire ; if further I consider, not so much the area which 

 is occupied by these remarkable rocks, but their mass, the fissures in 

 the crater-like hollow with vitrified sides, and such like, I cannot, on 

 the one hand, believe that there is nothing more in all this than con- 

 cretionary formations of the siliceous material ; and still less can I 

 believe that it is the effect of formerly-existing thermal springs, now 



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