Ch. XXVI.] 



CALLED e PRIMARY.' 367 



strata of pure siliceous sand alternating with micaceous sand 

 and with layers of clay, so in the * primary' we have beds of 

 pure quartz rock alternating with mica-schist and clay-slate. 

 As in the secondary and tertiary series we meet with limestone 

 alternating again and again with micaceous or argillaceous sand, 

 so we find in the ' primary 1 gneiss and mica-schist alternating 

 with pure and impure granular limestones. 



Passage of gneiss into granite If, then, reasoning from the 

 principle that like effects have like causes, we attribute the 

 stratification of gneiss, mica-schist, and other associated rocks, 

 to sedimentary deposition from a fluid, we encounter this diffi- 

 culty, that there is often a transition from gneiss, one of the 

 stratified series, into granite, which, as we have shown, is of 

 igneous origin. Gneiss is composed of the same ingredients as 

 granite, and its texture is equally crystalline. It sometimes 

 occurs in thick beds, and in these the rock is often quite 

 undistinguishable, in hand specimens, from granite; yet the 

 lines of stratification are still evident. These lines imply depo- 

 sition from water, while the passage into granite would lead us 

 to infer an igneous origin. In what manner can we reconcile 

 these apparently conflicting views ? The Huttonian hypothesis 

 offers, we think, the only satisfactory solution of this problem. 

 According to that theory, the materials of gneiss were originally 

 deposited from water in the usual form of aqueous strata, but 

 these strata were subsequently altered by their proximity to 

 granite, and to other plutonic masses in a state of fusion, until 

 they assumed a granitiform texture. The reader will be pre- 

 pared, by what we have said of granite, to conclude, that when 

 voluminous masses of melted rock have been for ages in an 

 incandescent state, in contact with sedimentary deposits, they 

 must produce some alteration in their texture, and this alteration 

 may admit of every intermediate gradation between that result- 

 ing from perfect fusion, and the slightest modification which 

 heat can produce. 



The geologist has been conducted, step by step, to this 



