412 



the veins and cavities. In this way a volcanic rock may become 

 impregnated with secondary silica in the form of hyalite, opal, chalcedony, 

 quartz, to such an extent that its bulk analysis will give a totally erroneous 

 impression as to its original composition. The andesites and porphyrites of 

 the Cheviot district furnish interesting illustrations of silicification. The 

 comparatively unaltered enstatite-andesite is traversed by veins of opal and 

 chalcedony (see Fig. 1, Plate XXX VI.), while the amygdaloidal porphyrites 

 are celebrated for their agates. In some cases the decomposition of a rock 

 by solfataric or other action may precede its silicification. Professor BLAKE, 

 has shown that portions of the Roche Castle Rock, Pembrokeshire, have 

 been highly silicified, so that the porphyritic felspars are now represented 

 by aggregates of quartz/ 1 ) In such a case as this it is clear that the 

 silicification has been preceded or accompanied by causes which have 

 decomposed the original rock, perhaps an andesite or trachyte, and 

 removed in a state of solution not only the alkalies, but also the com- 

 paratively insoluble alumina. The silicification of volcanic rocks by 

 thermal waters originating in connection with volcanic action must 

 naturally be associated in most cases with the formation of siliceous sinter. 

 Mr. MARK'S suggestion/ 2 ) therefore, that such masses of highly siliceous 

 rock (97'5/ of Si0 2 ) as those of Trefgarn were originally sinters is one 

 which deserves most careful consideration. 



As illustrating the importance of thermal waters, not only in 

 introducing new minerals into rocks, but also in forming mineral veins, 

 we may refer to the celebrated Steam Boat Springs of Nevada and 

 the associated ore-deposits. According to Mr. G. F. BECKER < 3 > the waters 

 of these springs are " now depositing gold, probably in the metallic 

 state ; sulphides of arsenic, antimony and mercury ; sulphides or sulpho- 

 salts of silver, lead, copper and zinc ; iron oxide and possibly also iron 

 sulphides ; manganese, nickel and cobalt compounds, with a variety of 

 earthy minerals. The sulphides which are most abundant in the deposits 

 are found in solution in the water itself, while the remaining metallic 

 compounds occur in deposits from springs now active, or which have been 

 active within a few years." Mr. PHILLIPS points out that these waters issue 

 from cracks and that the walls of the cracks are lined, sometimes to a 

 thickness of several feet, with incrustations of silica of various degrees of 

 hydration containing hydrated ferric oxide and occasionally crystals of 

 iron pyrites/ 4 ) 



Many igneous rocks, and especially those of basic composition, are 

 often highly charged with zeolites. The classic researches of M. DAU- 

 BREE< 5 ) on the formation of zeolites and other minerals in the Roman 



(1) Q.J.G.S., Vol. XL , 1884, p. 308. 



(2) See report of the discussion on Dr. BONNET'S paper. Q.J.G.S., Vol. XLII., 



1886, p. 363. 



(3) Natural solutions of Cinnabar, Gold and associated sulphides. A..T.S., Vol. XXXIII. 



1887, p. 199. 



(4) Ore Deposits, 1884, p. 70. 



(5) Etudes synthetiques de geologie experimental e, Paris, 1879, p. 180, 



