T. R. Holland — Roclc-iveatheying and Serpentinization. 543 



•certain alterations amongst the secondary minerals mentioned by 

 Professor Merrill.^ The formation of zeolites by the action of 

 thermal springs in brickwork was, everybody knows, considered by 

 Daubree to be a sufficient explanation of the origin of the amygdales 

 in volcanic rocks, and the same author suggested further that palago- 

 nite and serpentine were formed in a similar way.- Of the other 

 secondary minerals, without going into the extensive literature whick 

 exists, it will be necessary only to recall the pi'ocesses by which the 

 formation of kaolin, epidote, chlorite, biotite. and magnetite has 

 been traced out in detail by Professor Judd ^ in the propylites of 

 the western isles of Scotland, as the result of solfataric action on 

 previously fresh rocks of the andesitic and dioritic types, which are 

 the oldest of the Tertiary eruptives in that area. Professor Judd 

 carefully emphasizes a distinction between the widely-spread effects 

 of solfataric action and the somewhat similar, but purely local, effects 

 of contact-metamorphism. 



The other point raised by Professor Merrill, namely, the behaviour 

 of the water and other vapours included in the magma, has not, 

 I admit, been so generally recognized, and for that reason I have 

 recently taken advantage of more than one opportunity of bringing 

 the importance of this matter into prominence. In 1897 it was 

 pointed out that the water excluded from the mother-liquor by the 

 separation of anhydrous minerals from a basic aquo-igneous magma 

 may attack the susceptible ferromagnesian silicates previously 

 ■crystallized from the same magma, with the formation of secondary 

 biotite and magnetite.'' There are reasons for supposing that 

 a similar explanation applies to the zeolites of the Deccan Trap in 

 localities distant from any volcanic vent.^ 



But the idea, nevertheless, is not an original one ; for although 

 J. D. Dana® held in 1845 that amygdales were formed by the 

 infiltration of water, probably sea- water, he expressed a modification 

 of his views in 1880, stating " that the moisture which made the 

 amygdaloidal cavities was the moisture which altered the pyroxene 

 or other minerals of the rock to chlorite, and made the zeolites and 



1 "Rocks and Rock- weathering," pp. 161 and 174 (footnote). 



2 Quart. Journ. Geol. Soc, vol. xxxiv (1878), p. 82. 



3 Quart. Journ. Geol. Soc. vol. xlvi (1890), pp. 341-382. 



* " On Augite-diorites with Micropegmatite in Southern India " : Quart. Journ. 

 Geol. Soc, vol. liii (1897), p. 405. 



5 In a memoir on the Salem rocks, now in the press, evidence is given which 

 indicates an origin similar to this for the magnesite which is so constantly found 

 veining the peridotite masses in South India. Most, if not all, these peridotite 

 eruptions are accompanied by masses of white quartz containing liquid carbonic acid. 

 The association of these two rocks is too constant to be fortuitous, and is suggestive 

 of a genetic relationship, the quartz being more probably the siliceous end-product 

 of the eruption in each instance, and in the absence of alumina and alkalies con- 

 solidates as simple quartz instead of forming alumino- alkaline silicates. That being 

 so, the large quantities of carbonic acid under pressure might attack the olivine, with 

 formation of magnesite and separation of chalcedonic silica. The water associated 

 with the carbonic acid accounts for the small quantities of picrolite always found in 

 tliese peridotites, which, however, never show a general serpentinization of the mass. 



^ " Origin of the constituent and adventitious Minerals of Trap and the allied 

 Hocks" : Araer. Journ. Sci., vol, xlix (1845), pp. 49-64. 



