195 



It has been uplifted and protruded through sedimentary strata at 

 different periods, from the earliest to the latest age of igneous dis- 

 turbance. It may have been elevated in a solid state, or in a state 

 of partial or imperfect fusion. It may have changed its original 

 character, either by being heated a second time, and again cooling, 

 under circumstances different from those which attended its first 

 consolidation ; or by entering into fresh combinations with the rocks 

 above it, or with those beneath it. The further the rock is removed 

 from the reach of any such influences, the nearer does it approach 

 in character, in respect of its mechanical and chemical structure, to 

 true ternary granite. Even when granite has been so altered as to 

 assume the character of porphyry or trachyte, the original character 

 of the granite out of which those rocks were formed may often be 

 traced in the gneiss with which those rocks are flanked. 



The fine-grained varieties of ternary granite, which are often 

 found in veins, have probably been fused a second time. The seat 

 of the binary granites was probably below that of the ternary rock, 

 but higher than that of the granites which contain alkalino-earthy 

 substances. Granite containing other substances than quartz, felspar 

 (or alhite) and binaxal mica, has probably been again fused, and 

 has derived the foreign matters intermingled with it, either from the 

 sedimentary rocks through which it has been protruded, or from 

 regions below that of ancient ternary granite. Hence the variations 

 in modern granite are almost as numerous as the localities in which 

 they are found. Thus the granite of Devonshire and Cornwall, 

 which has been uplifted and protruded through all the stratified rocks 

 that were incumbent upon it, not excepting even the culm, was in a 

 fused state in its upper portion when in contact with those stratified 

 rocks ; and it probably brought up with it extraneous matters from 

 beneath. Hence the granite of these counties no longer exhibits the 

 characteristics of ancient granite ; but in some parts porphyritic 

 granite occurs ; the granitic dykes, or elvans, consist mostly of 

 eurite ; in other parts we have talcose granite, or protogine, which 

 produces the China clay ; and schorly granite is generally found 

 near the contact of that rock with the slates. 



The fact that ancient granite sometimes graduates into syenitic 

 granite, renders it probable that the latter is a modification of the 

 former. 



The substances which invaded the territories of ancient ternary 

 granite, were probably those which occupied the regions immediately 

 subjacent to it, while those which lay nearer to the earth's centre 

 remained comparatively undisturbed. 



In general, the conclusion of the author is, that the absence of 

 mica, or the presence of minerals abounding in magnesia or lime, 

 or that of metallic oxides, or a transition into syenite, porphyrj', 

 basalt, or volcanic rocks, are indications of an origin of later date 

 than that of ancient granite. 



