h. X.] ON THE PRIMARY ROCKS. 205 



the same external concretionary structure, so as often not to 

 be distinguishable at first sight, until the internal structure 

 be examined. It is in the massive kinds that the constituent 

 minerals are more distinctly displayed; indeed, in some 

 series, particularly those of Cornwall, they can only be de- 

 tected in these, and even then they are not always to be 

 discerned, except in minute portions or veins ; and thus it is 

 that these massive rocks experience transitions into granitic 

 rocks, not to be distinguished from those which enter into the 

 composition of the main masses of these rocks. In districts of 

 gneiss, for instance, the micaceous variety occurs in thick 

 laminae, which, becoming tabular and massive, cannot at last 

 be mineralogically separated from true granite ; or, the horn- 

 blendic variety, in the same manner, becomes massive syenite 

 or greenstone, perfectly resembling the corresponding mem- 

 bers of the granitic groups : so likewise the peculiar slates of 

 Cornwall, when massive, graduate into eurite, felspar-por- 

 phyry, shorl-rock, and other kinds, similar to those which 

 occur in the insulated masses of granite : and, lastly, it is no 

 uncommon occurrence for the granular and compact quartz- 

 rocks into which the primary slates often pass, and with 

 which they are constantly interstratified, to form an integrant 

 part of granitic groups, alternating with the different kinds 

 of the unstratified masses. 



Thus we learn that the granitic rocks, interspersed among 

 the primary slates, are intimately united therewith both in 

 composition and structure ; and the same holds good at the 

 junction of the main bodies of these rocks, as has been de- 

 tailed at some length in the seventh and eighth chapters: 

 and, indeed, a similar union subsists between limestone and 

 granite. At Glen Tilt, for example, its large grained crys- 

 talline texture gradually disappears as it approaches the 

 granite, and it more resembles hornstone or compact felspar ; 

 it effervesces slowly with acids, and gives, on analysis, a large 

 portion of silica : the same limestone passes into quartz-rock 

 by the same excess of silica, and this quartz-rock frequently 



