380 



PEOF. J. W. JUDD ON THE PKOPYLITES 



crystals being plagioclase. In this connexion it may be mentioned 

 that recent researches shaw that many of the rocks of Iceland 

 formerly regarded as rhj'olites must really be referred to the group 

 or tde very acid andesites. 



We have seen that the most glassy varieties of the Ben. Hiant 

 rocks have a very close analogy to the so-called " pitchstone- 

 porphyrj^ " of the Sgurr of Eigg. This latter rock, from the presence 

 of Carlsbad-twins, has usually been regarded as a ^^urely ortho- 

 clastic rock. But careful examination in polarized light often 

 proves that some of these felspars show undoubted evidence of 

 plagioclase-twinning. I have already remarked upon the fact that in 

 the glassy variation of these rocks the lamellar twinning of the 

 plagioclase often remains undeveloped. The quantity of glass 

 present as enclosures in these felspars would vitiate any result 

 obtained by their isolation and analysis. 



Nowhere can we find such clear evidence as in the rocks of Ben 

 Hiant of the truth of the conclusion that the phenocrysts of such 

 lavas as these were formed under Plutonic conditions, and that there 

 is no direct and necessar}' relation between the porphyritic crystals 

 of a volcanic rock and the magma by which they are enveloped. 



In the case of the glomero-porphyritic rocks, I have show^n that 

 the evidence points to the existence of a holocrystalline mass having 

 been broken up and its fragments enveloped and carried up in a 

 magma of different composition. In certain pitchstones from Colo- 

 rado, for specimens of which I am indebted to Mr. Louis, I have 

 found fragments of micro-pegmatitic rocks enveloped in a perfectly 

 glass}' magma. In hornblende-andesites from Auvergne I have 

 found glomero-porphyritic fragments of an enstatite-andesite, and 

 in the case of the Krakatoa lava I have shown that the crystals 

 are not scattered at random, but really form groups derived from 

 a preexistent nearly holocrystalline mass. 



Now some of the porphyritic crystals of the Ben Hiant rocks 

 show all those features which I have already pointed out as being 

 characteristic of deep-seated rocks. Both the felspars and the augites 

 show incipient schillerization, and this is found to be the case even 

 in crystals enveloped in a perfectly glassy and vesicular matrix. 



In the face of all these facts, I believe that it will be found 

 impossible to maintain that the porphyritic constituents of such 

 rocks as these could have been formed except under Plutonic condi- 

 tions ; and their present condition of corrosion and partial resorp- 

 tion proves that, in the volcanic masses as poured- out on the earth'a 

 surface, these materials of the Plutonic consolidation are in a con- 

 dition , of instability — very different indeed from that in which 

 they were originally formed. 



[Since the reading of this paper I have had the opportunity of 

 studying the remarkable work of Mr. G. F. Becker on the rocks of 

 California. He has shown that, closely associated with the ande- 

 sites and basalts of the district, are great masses of glass, contain- 

 ing few crystals or none at all, and presenting therefore a higher 



