160 p. EUTLEY ON BEECCIATED POREIDO-EOSSO AlyTICO. 



and to the subsequent cementing of the crushed material in situ. 

 The finely comminuted porphyrite-substance which lies in the cement 

 tends to prove this. 



With regard to the power of resistance to crushing which this 

 rock possesses, Delesse states that, compared with that of the red 

 Egyptian syenite, it is as 2| to 1. 



The ground-mass of the porphyrite-fragments, when magnified 

 250 linear, shows great numbers of minute granules and a smaller 

 proportion of microliths. Most of the latter appear to lie with their 

 lougest axes parallel, from which it may be inferred that they 

 are normal constituents of the porphyrite and not devitrification- 

 products. The devitrification seems for the most part to be of a 

 granular and not of a microlithic character. 



Although the veins of cementing matter contain some felspar 

 among the other finely crushed material derived from the porphyrite, 

 I am inclined to regard it mainly as quartz. Delesse describes 

 specimens of this rock in which very distinctly visible quartz-veins 

 occur, one which he figures being certainly not less than an eighth 

 of an inch in breadth at the broadest part. He states, however, that 

 quartz is of rare occurrence in the Porfido-rosso antico. The same 

 author observes, with regard to the constituent minerals of the rock 

 that the felspars are tricHnic and have a composition which is 

 between that of oligoclase and andesine, that hornblende is present, 

 and also specular iron and magnetite, the rock itself possessing 

 a feeble magnetic power. 



Prom a microscopic examination it appears that the felspars are 

 much altered, in many cases containing what appears to be epidote. 

 Posenbusch describes a reddish alteration-product in the felspars of 

 this rock, which he also is inchued to regard as epidote. He like- 

 wise mentions the occurrence of apatite*, of which some good 

 crystals may be seen in the section here described. The magnetite 

 occurs in octahedra, and also in irregularly shaped patches. I have 

 been unable to get satisfactory measurements of the angles of 

 extinction in the felspars, some crystals giving very small angles, 

 while others extinguish at about 40° from the planes of composition 

 of the twin lamellae, and these must in all probability be anorthite. 

 The majority, however, have low extinction- angles and are most 

 likely oligoclase or andesine, as suggested by Delesse. . 



The hornblende-crystals are very perfectly developed, and few 

 rocks contain more tj'pical examples of this mineral. In one 

 Id stance a hornblende crystal contains in its centre an irregular 

 enclosure of quartz, and many of them envelope magnetite, either 

 in octahedral crystals or in irregular grains. 



The Porfido-rosso antico occurs at DjebelDokhan in a vein from 20 

 to 25 metres broad, traversing pegmatite. Particulars concerning its 

 mode of occurrence are given in the paper by Prof. Delesse already 

 cited. Some sections lent to me by Prof. Judd show here and there 



* ' ]Mikroskopi3che Physiographie d. massigen Gesteine,' p. 290 : Stuttgart, 

 1877. 



