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The strata are much hidden by detritus, but they appeared in most parts to be highly inclined; and in an adjoining lofty pinnacle they could be distinctly seen bending up, and becoming vertical, conformably with the underlying porphyritic conglomerate.
At a re-entering angle of the junction, a shallow pit was sunk; the sand became moist and red, and presently it was underlaid by a rubble of porphyritic trap. Nothing more! We then crossed the Wady Rabigh, another of the short broad valleys which distinguish this section of South Midian.
The degradation of the rock on which this formation rests is constantly bringing down masses of it, and mingling them with the basaltic, porphyritic, granitic, and calcareous fragments which the torrents carry down to the valleys, and, through them, in a state of greater or less disintegration, to the sea.
The same lava, for example, may be glassy, or scoriaceous, or stony, or porphyritic, according to the more or less rapid rate at which it cools. It would be easy to multiply examples and authorities to prove the gradation of the Plutonic into the trap rocks.
Thus the terms Trachytic porphyry, Trachytic tuff, etc., merely refer to the same rock under different conditions of mechanical aggregation or crystalline development which would be more correctly expressed by the use of the adjective, as porphyritic trachyte, etc., but as these terms are so commonly employed it is considered advisable to direct the student's attention to them. Porphyry.
I believe this last great bed is covered by other nearly similar alternations; but the section is here obscured by a tilt from the next porphyritic chain, presently to be described. I have given this section in detail, as being illustrative of the general character of the mountains in this neighbourhood; but it must not be supposed that any one stratum long preserves the same character.
In the neighbourhood of the above-mentioned road from Koptos to Berenicê were the porphyritic quarries and the emerald mines, which were briskly worked under the Emperor Claudius. The mountain was now named the Claudian Mountain. As this route for trade became known, the geographers began to understand the wide space that separates India from Africa.
Some miles up the harbour, in a line of cliffs formed of slightly metamorphosed tufaceous and porphyritic claystone beds, I observed three vertical dikes, so closely resembling in general appearance ordinary volcanic dikes, that I did not doubt, until closely examining their composition, that they had been injected from below.
In the more northern parts of Chile, this porphyritic series extends over large tracts of country far from the Cordillera; and even in Central Chile such occasionally occur in outlying positions. I will describe the Campana of Quillota, which stands only fifteen miles from the Pacific, as an instance of one of these outlying masses.
The largest links of these West-Arabian Ghats are of white-grey granite, veined and striped with quartz; and they are subtended inland by the porphyritic traps of the Jibal el-Shafah, which we shall trace to the parallel of El-Hamz, the end of Egypt. All are maritime, now walling the shore, like the Tayyib Ism; then sheering away from it, where a broad "false coast" has been built by Time.
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