26 THE MINERAL CONSTITUENTS OF ROCKS. 



took pLice in an old vent, from Avhich the plug of lava and ashes must be 

 removed before the outflow could occur. 



The action of lavas upon these foreign inclusions seems to be that of a 

 corrosive dissolving hot magma or solution, which penetrates and gnaws its 

 way into the included fragments. Two classes of foreign materials seem to 

 be characteristic of most of the eruptive rock species — simple minerals, and 

 rock fragments. The latter are either the same as the inclosing rock or else 

 they are the same as some rock known to have reached the earth's surface 

 earlier in order of time. These mineral inclusions characterize the same 

 rock type from the earliest times to the present, when- and where-ever they 

 may occur. All this indicates some deep seated universal cause beyond the 

 influence of sedimentarv rocks. 



These characteristic minerals, too, are not such as occur in sedimentary 

 rocks; while no such admixture of material exists in the eruptive forms as 

 w^ould naturally be expected to occur if they were formed from sediments. 

 Then too, the minerals and fragments of tlie more difficultly altered sedi- 

 mentaiy rocks ought to have remained side by side with these easily alter- 

 able foreign minerals in eruptive rocks, if the latter rocks are the re-fused 

 portions of the former. The microscopic characters of the eruptive rocks 

 are to my mind utterly opposed to any theory that they come from sedi- 

 ments, or anything else than the original liquid material of the earth. 



The second class of rock constituents naturally occupies the most promi- 

 nent place in recent volcanic rocks, and a more subordinate one in the older 

 eruptive and sedimentary ones. In eruptive rocks, the indigenous materials 

 are the products of the magma when unacted upon by extraneous agencies. 

 It is doubtful if any minerals come under this head — direct jirimary pro- 

 ducts of crystallization of the magma — excejit anhj'drous silicates and 

 oxides, a few phosphates, sulphides, and native elements, the other minerals 

 in the rocks belonging to the other two classes. 



The third class becomes very prominent in the older and altered rocks, 

 and includes the hydi-ous and some anhydrous oxides and silicates, carbon- 

 ates, and most sulphides. 



These forms are the products both of alteration taking place in the rock 

 mass and of material brought into the rock from extraneous sources. In 

 one case the chemical constitution of the rock remains essentially unim- 

 paired, while in the other that constitution is changed to a greater or less 

 degjree. The causes of these alterations in ancient and modern volcanic 



