what is the explanation given by geologists about the Nile valley
Answers
(i) The saturation with silica of the old sand-and-gravel beds overlying the Nummulitic Limestone near Cairo, forming sandstones of all qualities from an extremely hard stone to one that can be rubbed down to powder between the fingers.
A similar saturation, apparently from water containing silica in solution, is visible in the beds overlying the Nummulitic Limestone in Biluchistan.
(ii) The fact that these old sands and gravels are formed from very old rocks, principally granitic. They contain great quantities of the Assouan pebbles, brown and red, wholly or semi-transparent, which were so largely used by the Romans to cut cameos and intaglios. Some minerals are found in these gravels, notably cobalt, the source of which is unknown.
(iii) The petrified forests of the same region.
(iv) The occurrence in greater or less quantity of titaniferous magnetic oxide of iron in the beds forming the Nile Valley;
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