ME. F. ETTTLEY ON THE ALTEEATION OF [Feb. 1 896, 



5. On the Alteeation of certain Basic Eetjptive Rocks from 

 Beent Toe, Devon. By Fbank Rutley, Esq., F.G.S. (Read 

 December 4th, 1895.) 



[Abstract.] 



Two microscopic sections of rocks occurring on the north side of 

 Brent Tor were examined, and a cursory glance suggested at once the 

 idea that they might originally have consisted to a greater or less 

 extent of extremely vesicular basalt-glass. No unaltered vitreous 

 matter, except perhaps mere traces, can now be detected in these 

 specimens, the interest of which lies in the assemblage of alteration- 

 products which they contain. A third section cut from a small chip 

 collected at the southern side of the base of the Tor consists of a highly 

 vesicular lava of a hyalopilitic character, which may be regarded as 

 an amygdaloidal glassy basalt. 



The Author gives a detailed account of the microscopic characters 

 of the three sections, and discusses the history of the rocks, com- 

 paring them with Tertiary basic glass, and with the Devonian 

 rocks of Cant Hill, which he described previously. He brings 

 forward evidence in favour of the view that the original alteration 

 of both the Brent Tor and Cant Hill rocks was palagonitic, and 

 that while in the Brent Tor rocks the subsequent alteration of the 

 palagonite into felsitic matter, magnetite, secondary felspar, epidote, 

 and probably kaolin, and some serpentine and chlorite was com- 

 plete, it was only partial in the case of the Cant Hill rocks. "We 

 may therefore assume that palagonite is not the ultimate phase of 

 alteration in basic igneous rocks. 



Discussion. 



Gen. McMahon enquired whether the Author had worked out in 

 detail the chemical changes involved in the conversion of palagonite 

 into ' felsitic matter ' and ' secondary felspar.' He understood the 

 Author to state, and the illustrations exhibited seemed to imply, that 

 palagonite was changed in situ into ' felsitic matter ' without the 

 addition of any extraneous material ; but as the chemical composition 

 of palagonite, as seen in the table on the wall, only showed a total of 

 8*60 per cent, of aluminium oxide, some explanation as to how this 

 change could be brought about seemed necessary. 



Prof. Bonnet said that he thought most of the Author's con- 

 clusions would not be disputed, for the changes which had been 

 described had many precedents, though he doubted whether it was 

 possible to trace the mineral alteration of palagonite until it was 

 more precisely denned what is palagonite. But he agreed with 

 Gen. McMahon in thinking that ' felsitic matter ' could not be 

 formed from an ordinary palagonite, unless we assumed chemical 

 changes which were not probable. 



