304 Doubts Relative to the Epochal and 



36 per cent, of magnesia. It, therefore, cannot be supposed 

 that the hornblende in the granite, which is pre-occupied in 

 forming the hornblende schist of the gneiss system, can also 

 suffice to yield in addition as much lime and magnesia as 

 occur in the dolomite and talc schist of the same system ; 

 and still less that it should, beyond that, suffice to account 

 for the lime occurring as carbonate of lime in the primary 

 limestone of the gneiss system. 



Again, even supposing that the hornblende schist has had 

 both magnesia and lime abstracted from it, and in places 

 here and there it be found to be considerably freed from 

 them, yet the amount of hornblende or syenite is so incon- 

 siderable as compared with quartz, mica, and felspar, that 

 it is totally insufficient to supply the amount of crystalline 

 limestone and of lime contained in dolomite found in the 

 metamorphic rocks ; and the mere fractional amount con- 

 tained in felspar is scarcely worth a serious consideration in 

 accounting for so large an amount of lime as is found in the 

 primary limestone and dolomite belonging to the gneiss 

 system. 



If then lime, as contained in granite, is more than ex- 

 hausted in supplying the first series of the transitionary 

 rocks, and that alkaline earth is there taking precedency 

 over its representative in granite, what can be said of lime 

 in the strata above them ? Passing over the Skiddaw 

 system of rocks, in which hornblende and chiastolite slate 

 are largely supplied with magnesia rather than lime, we 

 come to the Cambrian and Silurian systems. In these 

 rocks, whether the Bala or the Llandeilo, the Plinlimmon 

 or the Wenlock rocks be examined, there we find LIME in 

 considerable abundance, and among the alkaline earths 

 taking the foremost position. From the silurian onwards, 

 through the Devonian, carboniferous, new red sandstone, 

 on to the chalk, lime is found amongst the alkaline earths in 



