F. ETJILEY OX BRECCIATED PORFIDO-ROSSO ANTICO. 159 



position rather than fusion. The ground-mass of the porph3Tite 

 fragments, although possessing a finely granular or crypto-crystalline 

 structure, appears very dark during rotation between crossed nicols ; 

 and from this it seems probable that it was once vitreous, and that 

 there is still more or less of a glassy residuum. 



The following extract from Delesse's paper shows that he was 

 •well acquainted with the brecciated character of this reck, although 

 he could not then have seen it in the perfection with which it is 

 demonstrated in microscopic sections : — " The structure of the 

 Porfido-rosso antico is seldom completely uniform; it is often 

 brecciated, as is the case in other porphyries, and especially in the 

 melaphyres ; the angular fragments which it contains are therefore 

 simply varieties of the rock itself. These fragments are sometimes 

 quite distinct from the paste which surrounds them, and from which 

 they are sharply separated. Sometimes, on the other hand, they 

 have no definite contours, and they melt imperceptibly into the 

 porphyry." (A figure in illustration of this is appended to his 

 paper, in which the brecciated character is merely indicated by the 

 nesting together of the felspar crystals in irregular areas.) " One 

 sees, in fact, that certain parts having angular forms are quite 

 distinct from the paste, for they take a fine polish, have a slightly 

 different colour, and contain larger and better-formed crystals of 

 felspar. It is, however, impossible to distinguish the boundaries of 

 porphyry-fragments corresponding to these angular patches ; for it 

 may be that they are fragments which have been fused, or stuck 

 together {ressoucles)^ or that, the paste not having been brought to 

 a state of complete fluidity and not having a uniform composition, 

 the felspar was unequally developed in the rock." Prom a careful 

 examination of the section already described, I do not think that 

 this latter theory is tenable ; and although the former might at first 

 sight appear to be true, yet the decomposed character of the borders 

 of the fragments contradicts such an hypothesis. As a rule, the 

 fragments of a rock which has been crushed and recemented in situ 

 would, if brought in contact, fit together accurately like the pieces 

 of a puzzle. In the section before us there is a general correspondence 

 of the disrupted parts, although, in some places, the fragments have 

 become widely separated and possibly shifted slightly in a rotatory 

 manner; while the alteration of some of the porphyritic felspar 

 crystals into micro-felsitic matter, closely resembling the cement, 

 tends, especially when such crystals lie on the borders of the frag- 

 ments, to increase the impression that the neighbouring fragments 

 would not fit together. 



If the fragments were volcanic ejectamenta, there would be little 

 or no appearance of their former continuity. Moreover, in this rock 

 there is not the slightest admixture of material derived from other 

 rocks, while in volcanic tuffs and dust one frequently finds that the 

 constituent grains and lapilli are not all of one kind. Having, in 

 the first instance, assumed, from the general appearance of the 

 section, that the rock was a tuff, I have been gradually led to the 

 conviction that its brecciated character is due simply to crushing 



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