FOOTNOTES:
[1]In geology, the term rock is applied alike to the soft and hard materials composing the earth’s crust, as sands may pass into solid sandstones, soft muds become the most tenacious of clays, and hard basalt originate as a molten lava.
[2]This statement may not absolutely apply to Cairo itself, seeing that in the disturbed region of Abu Roash sandstone is actually at the surface, and the clays are less conspicuously developed than is the case to the south.
[3]Figs. [1,] [2] and [4] are based on photographs of localities in Great Britain and Ireland taken for the Geologists’ Association collection of photographs.
[4]Farafra is an exception to this rule, the depression being due to the wearing away of soft Cretaceous beds higher than the sandstone.
[5]The well-known Petrified Forests.
[6]From information given by Mr. Harding King.
[7]At Abu Radham a dark layer of purple iron ochre seems to have been the cause of the investigation.
[8]For further details, see my paper “Petroleum: Its occurrence and origin,” Cairo Scientific Journal, No. 48, pp. 205-218.
[9]“The Natural History of Igneous Rocks,” p. 23.