356 GEOLOGICAL APPEARANCES 



Sir James Hall mention the aggregation of curd and jelly, as 

 an analogy to obviate similar difficulties. Perhaps the pene- 

 tration of a fragment still retaining its shape may in some 

 cases have been effected, by one more soluble component 

 having been withdrawn by the action of a fluid in contact, and 

 having thus left the pores it occupied to be filled with a new 

 substance, derived from the fluid. It is clear, however, that 

 Avhere there is gradation on the lines of junction, it has been 

 owing in some cases to a trifling difference of circumstance ; 

 for, among the specimens procured by a blast at D, the lines of 

 junction are precisely marked in some, while in others the red 

 felspar appears disseminated through the same blackish strati- 

 fied compound for an inch, or more, from the side of the 

 vein. 



131. An igneous theory for the sienite might lead us also to 

 ascribe the high state of crystallisation, in some of the stratifi- 

 ed masses, to a slow reconsolidation after having been soften- 

 ed by the heat. Sir James Hall's experiments upon the fu- 

 sion of carbonate of lime under compression, make it more 

 easy to admit this hypothesis with respect to the limestone, 

 than in the case of the gneiss, and some other substances. 

 The important information respecting the properties of carbo- 

 nate of lime, which we have derived from Sir James Hall's 

 experiments, may assist us in explaining some other appear- 

 ances in these rocks. Thus we may conceive that the vein of 

 limestone proceeding from an imbedded mass, (par. 82.) was 

 produced from a part of the mass which had been completely 

 melted, and remained so, after the sienite had grown so hard 

 as to admit of a crevice being formed. A similar origin may 

 be ascribed to the vein of hard granular limestone at C, 

 (par. 60.). The veins of calcareous spar, cutting strata of 

 gneiss, (par. 96.) may have been formed, either from fused por- 

 tions 



