Lieut. Newbold on the Geology of Egypt. 215 
be found very useful. In one specimen which I have ex- 
amined, having its faces inclined 2° or 3° to each other, I can 
distinctly see the line D of the spectrum formed from the light 
ofa candle with a salted wick; and I have no doubt that speci- 
mens of agate will be found, and may be so nicely prepared in 
extremely thin plates, with their surfaces perpendicular to the 
veins, as to give diffraction spectra more perfect, and much 
more enlarged than it is possible to obtain from any system of 
parallel grooves that can be produced by art. 
To the mineralogist this determination of the structure of 
the agate cannot fail to be interesting. ‘The difference in the 
colour of the veins and their intervals, and their singular 
equality of thickness, is very remarkable. In the structure of 
mother-of-pearl, the succession of strata or veins marks the pe- 
riod of rest during which the animal has ceased to labour; and 
in the structure of nacrite, the artificial mother-of-pearl formed 
upon the dash-wheel at the cotton-works at Catrine*, the 
passage of one stratum into another indicates the daily rest of 
the wheel, and of the operations to which it gives rise; but it 
is not easy to understand how an aqueous solution of silex 
contained in the cavity ofa solid rock, should deposit its solid 
contents with that uniformity and regularity which are found 
in structures depending on the periods of animal life or human 
Jabour. 
St. Leonard’s College, St. Andrew’s, 
February 13, 1843. 
XXXVII. On the Geology of Egypt. By Lieut. Newzoxp, 
ERS. of the Madras Army +. 
M® Newbold first describes the physical features of Egypt, and 
2ndly, the formations of which the country is composed. 
I. Physical Features.—After alluding to the natural boundaries of 
Egypt, namely, the Mediterranean on the north, the Libyan desert 
on the west, the mountains of Nubia on the south, and the Red Sea, 
with the Isthmus of Suez, on the east, and stating that the area thus 
circumscribed comprises about 100,000 square miles, the author shows 
that Egypt has three great physical divisions: 1. the mountainous 
region extending between the Red Sea and the Nile; 2. the deserts 
east and west of the Nile; and 3. the fertile valley of that river, with 
its delta. 
The mountainous region is naked and dreary in aspect, on account 
of the deficiency of springs, rain, and dew; and it presents bare or 
* Phil. Trans,, 1836, p. 49, and this Journal, S. 3. vol. x. p. 201. 
+ From the Proceedings of the Geological Society, vol. iii. part 2, being 
an abstract of a paper read June 29, 1842, 
