APPENDIX. 473 



those circumstances of contact and of veins by which 

 the relative ages of any masses may he inferred; with, 

 of course, its connections and contacts with adjoining- 

 strata, and the several phenomena of its veins. And as the 

 varieties of this rock are highly interesting, especially 

 under its passages into trap, a correct description of 

 its mineral characters is necessary. And I may add a 

 remark applicable to all rocks, viz. that the geologist 

 should ever bear in mind the various opinions on these 

 subjects, whatever he rnay himself believe: since these 

 will sharpen his attention, and direct him to much that 

 might otherwise escape his notice. 



The antient porphyries having been generally ranked 

 with the stratified substances, we have been much 

 misled by their descriptions; but the same rules may 

 be adopted for these as for granite and the traps : taking 

 care especially to note those circumstances by which 

 their ages may be inferred, in reference to granite and 

 to the associated strata. In the trap rocks, the same 

 general rules apply, as far as all the circumstances of 

 position, extent, general forms, structures, mineral 

 characters, relations to the strata, junctions, and veins, 

 are concerned. Here also it is requisite to distinguish 

 between successive formations and their relative ages; 

 while, as masses, now independent, have often been 

 formerly connected, the geologist must bear in mind 

 all these facts, as already described, and also recollect 

 that the antient existence of even extensive tracts can 

 often be inferred by only the most delicate evidence. 

 Thus do their present connexions with the stratified 

 rocks demand the most careful attention, and often 

 much laborious comparison. On their mineral cha- 

 racters I should add, that they will often require acute 

 discernment respecting the distinctions of mixed mine- 

 rals, as may their intermixtures of character; yet the 



