F, EUTLEY ON BRECCIATED PORFIDO-ROSSO ANTICO. 157 



20, On Brecciated Porfido-rosso axttco. By Frank Hutlet, 

 Esq., F.G.S,, Lecturer on Mineralogy in the Xormal School of 

 Science and Eoyal School of Mines. (Read February 25, 1885.) 



This well-known rock, the porphyrites of Pliny, the typical porphyry 

 of the earlier, and one of the typical hornblende-porphyrites of 

 more recent petrologists, has already been described in considerable 

 detail by numerous writers, especially in an admirable paper by 

 the late Prof. Delesse*. His observations upon a large number of 

 specimens collected at Djebel Dokhan, the Porphyrites Mons of the 

 ancients, by M. Lefebvre, are remarkable for the precision with 

 which they were made at a period when the microscopic examina- 

 tion of thin sections of rock was unknown, or at all events in the 

 very year (1850) when such sections were first made by Dr. H. C. 

 Sorby. 



The observations of Delesse were made chiefly with low magni- 

 fying-powers upon polished surfaces of the rock, and by reflected 

 light. Brief descriptions of it, based on the examination of thin 

 sections, have also been published in several works. On looking at 

 a section recently prepared from a specimen of this rock, I was 

 surprised to see that it consisted, not of a continuous section of 

 hornblende-porphyrite, but of fragments of that rock held together 

 by a micro-crystalline granular cement. On referring to Delesse's 

 paper, I found that he described two kinds of the Porfido-rosso 

 antico, one being the rock in its normal condition, the other a 

 brecciated variety. It is to the latter that I would now direct 

 attention. 



The specimen is of reddish-brown or maroon colour, flecked with 

 small white or reddish- white crystals of felspar and dark hornblende 

 crystals, which as a rule are considerably smaller than the felspars. 

 It closely resembles the fig, 2 on the coloured plate attached to 

 Delesse's paper. The section, when examined under a low power by 

 ordinary transmitted light (see figure, p. 158), is seen to consist of 

 angular and subangular fragments of hornblende-porphyrite ranging 

 from about an eighth of an inch in diameter to very small dimen- 

 sions, and surrounded in most cases by opaque borders of what 

 appear to be dark granules. This bordering of the fragments and 

 their frequently well-rounded angles suggest, at first sight, the 

 idea that they are lapilli which have undergone superficial fusion. 

 That they are true fragments there can be no doubt ; for the horn- 

 blende and felspar crystals which lie on their margins are often 

 irregularly broken away, while a fine debris of hornblende crystals, 

 felspar crystals, and porphyrite ground-mass is scattered through 

 the micro-crystalline cement which binds the fragments together. 

 The ground-mass of the porphyrite looks as if it were crypto- 



* "Eecherches sur le Porphyre Eouge Antique et sur la Syenite Rose 

 d'Egypte," Bull. Soc. Geol. de France, 2« serie, t. vii. 



Q. J. G. S. No. 162. N 



