On the Genesis of Some Scottish Minerals. 207 



invaded. Such a solution, continuing to act at a moderately 

 high temperature, gradually dissolves the materials of the 

 rock, while the fluidity of the magma favours the equalisation 

 of its composition by convection and diffusion throughout 

 the whole of the part affected. After a time, the conditions 

 begin to change, the temperature falls, the aqueous solvent 

 mostly escapes, the various constituents of the magma 

 gradually crystallise out, each in the order determined by the 

 pressure and the temperature proper for the crystallisation, 

 and in course of time the whole passes into the condition of 

 a solid mass of rock. On this view of the origin of eruptive 

 rocks their plutonic and trappean forms may be regarded as 



PSEUDOMORPHS. 



It should be noted that the change of state from solid to 

 liquid, in the case of most of the rock-forming minerals, takes 

 place at definite temperatures and abruptly ; so that the line 

 between the dissolved and the undissolved rock must, on 

 the whole, be usually well defined. At the same time, it is 

 quite conceivable that some of the constituents might pass 

 from the solid into the fluid condition sooner than others. In 

 this way one can easily account for the introduction of certain 

 minerals into the rock invaded, and also for that solution and 

 redeposition of silica which has converted sandstones into 

 quartzites, and shales into Lydian stone, in the contact zones 

 of eruptive rocks. It has long been known that heated 

 waters charged with alkalies are capable of producing upon 

 coal-seams all the effects seen in the case of their contact 

 with intrusive masses, and that they are able to do this at 

 a comparatively low temperature. The " burnt coal-seams " 

 of the miners are, therefore, not necessarily due to dry 

 heat. 



An impartial consideration of these theoretical views 

 regarding the origin of the eruptive rocks will serve to 

 show that they explain many other facts observed in the 

 field besides these above referred to, and that they will also 

 satisfactorily account for many of the phenomena of both 

 contact and dynamo metamorphisra, to be referred to in more 

 detail presently. 



VOL. XIV. 



