582 PROTRUSION OF SOLID GRAJSTITE. [Ch. XXXIY 



gneiss is seen occasionally, on the removal of the newer beds, containing 

 organic remains, to be worn and smoothed ; secondly, pebbles of gneisa 

 have been foimd in some of the Silurian strata. Between the origin, 

 therefore, of the gneiss and the granite there intervened, first, the period 

 when the strata of gneiss were denuded ; 2dly, the period of the deposition 

 of the Silurian deposits. Yet the granite produced, after this long interval, 

 is often so intimately blended with the ancient gneiss, at the point of 

 junction, that it is impossible to draw any other than an arbitrary line 

 of separation between them ; and where this is not the case, tortuous 

 veins of granite pass freely through gneiss, ending sometimes in threads, 

 as if the older rock had oficred no resistance to their passage. It seems 

 necessary, therefore, to conceive that the gneiss was softened and more 

 or less melted when penetrated by the granite. But had such junctions 

 alone been visible, and had we not learnt, from other sections, how long 

 a period elapsed between the consolidation of the gneiss and the injec- 

 tion of this granite, we might have suspected that the gneiss was scarcely 

 solidified, or had not yet assumed its complete metaniorphic character, 

 when invaded by the plutonic rock. From this example we may learn 

 how impossible it is to conjecture whether certain granites in Scotland, 

 and other countries, which send veins into gneiss and other metamorphio 

 rocks, are primary, or whether they may not belong to some secondary 

 or tertiary period. 



Oldest granites. — It is not half a century since the doctrine was very 

 general that all granitic rocks were primitive, that is to say, that they 

 originated before the deposition of the first sedimentary strata, and 

 before the creation of organic beings (see above, p. 9). But so greatly 

 are our views now changed, that we find it no easy task to point out a 

 single mass of granite demonstrably more ancient than all the known 

 fossiliferous deposits. Could we discover some Lower Cambrian strata 

 resting immediately on granite, there being no alterations at the point 

 of contact, nor any intersecting granitic veins, we might then affirm the 

 plutonic rock to have originated before the oldest known fossiliferous 

 strata. Still it would be presumptuous, as we have already pointed out, 

 p. 452, to suppose that when a small part only of the globe has been 

 investigated, we are acquainted with the oldest fossiliferous strata in the 

 crust of our planet. Even when these are found, we cannot assume that 

 there never were any antecedent strata containing organic remains, which 

 may have become metamorphic. If we find pebbles of granite in a con- 

 glomerate of the Lower Cambrian system, we may then feel assured that 

 the parent granite was formed before the Lower Cambrian formation. 

 But if the incumbent strata be merely Silurian or Upper Cambrian, the 

 fundamental granite, although of high antiquity, may be posterior in date 

 to known fossiliferous formations. 



Protrusion of solid granite. — In part of Sutherlandshire, near Brora, 

 common granite, composed of felspar, quartz, and mica, is in immediate 

 contact with Oolitic strata, and has clearly been elevated to the surface 



