106 ANNALS NEW YORK ACADEMY OF SCIENCES 



have been metamorphosed in places to a tourmaline hornfels,^'' while in 

 the sill itself fissures are lined with nests of datolite. 



Metallic sulphides, pyrite and chalcopyrite especially, are often observ- 

 vible among the secondary minerals but are rarely seen in the unaltered 

 basalt. Although present in but small amount, there was undoubtedly 

 some concentration of sulphides in and near the channels of circulation. 

 This may likewise be accounted for by the emanation of sulphur com- 

 pounds and sublimates of copper and iron given off by the magma. 

 Covellite (CuS) has been found on the lavas of Vesuvius,^^ and atacamite 

 (CuClg, 3Cu(OH)2) is also found in crevices in the A'esuvian lavas, as 

 well as the chlorides of iron.^^ Among the fumarolic deposits in the 

 crater of Vulcano,-^ which are partly sul:)limates and partly secondary 

 products, A. Bergeat gives compounds of cobalt, zinc, tin, lead and copper. 

 Sulphides have also been found as sublimation products at Vesuvius, 

 formed perhaps by the action of HoS upon volatilized metallic chlorides. 



The small amount of fluorine represented by apophyllite may also be 

 referred to the magma. Fluorite sometimes appears on volcanic lavas.'* 

 It is possible, however, that the crusts of glass retained sufficient fluorine 

 to supply the very moderate amounts required to account for the forma- 

 tion of apophylite. 



Such sublimates given off by the consolidating AVatchung lava were 

 probably deposited in crevices and interstitial spaces upon a slight fall of 

 temperature. The places favorable for deposition were the very ones 

 which would be easily reached later by circulating waters. 



The aqueous contents of the lava would be completely expelled into the 

 open air. The source of the waters which effected alteration must there- 

 fore be sought elsewhere. A theory of derivation from meteoric sources 

 by a process of underground circulation appears to fulfill the require- 

 ments of the situation and to offer no obstacle. The preexistent topo- 

 graphic depressions in the areas occupied by lakes, although buried by the 

 flow of lava, would nevertheless be reached by waters flowing in from the 

 edge of the sheet through the porous sedimentary strata lying just beneath 

 the impervious mass of normally crystallized igneous rock. Having 

 reached such depressions, the hydrostatic head would be that due to the 

 difference in elevation between the depressions and the point of ingress. 

 The somewhat loose and porous structure of the accumulations of lava 



20 Andrew and Osann : Verb, des naturhist.-med. Ver. zu Heidelberg, n. f., vol. 1, 1892. 



21 E. S. Dana : "System of Mineralogy," 6th edition, p. 68, 1893. 

 «• Ibid., p. 165. 



23 F. W. Clarke : Op. cit., p. 220, ff. 

 2* F. W. Clarke : Ibid., p. 274. 



