Ch. XIV.] OF THE STRATIFIED ROCKS. 3 1 7 



is very crystalline, not to be distinguished mineralogically 

 from granite, and which, at a distance therefrom, though re- 

 taining the same component parts, is more perfectly stratified, 

 and not so abounding in granite-veins. 



How comes it to pass, it may be asked, since heat alone 

 can produce gneiss from the sedimentary strata, that this 

 primary schist is not always found in contact with granite ? 

 It may be answered, that the degree of intensity of the melted 

 mass varied, and that other schists were produced when the 

 temperature was not sufficient to form gneiss. This, how- 

 ever, would be mere conjecture; but it might be more 

 plausibly urged, that the nature of the sedimentary rock 

 induced a corresponding difference in the metamorphic rock : 

 indeed, it has been stated, " that, as in the secondary form- 

 ations, we find an indefinite series of clay, marl, sand, and 

 limestone ; so, in the primary, gneiss, mica-slate, hornblende- 

 schist, quartz-rock, and marble, have no invariable order of 

 superposition." This explanation, however, is more specious 

 than correct ; for all the primary slates, associated together in 

 every possible combination, pass into each other so frequently 

 by the most imperceptible gradations, and appear to be 

 ultimately composed of the same elements, that it is difficult 

 to suppose that they have been individually produced from 

 such dissimilar substances as clay, marl, sand, and other de- 

 posits. To render this objection more obvious, we must refer 

 to the primary slates generally. Clay-slate is said to be 

 derived from shale, and when clay-slate comes into contact 

 with granite, it passes oftentimes into hornblende- schist, 

 which has been adduced as evidence of a more perfect igneous 

 change, bearing the same relation to clay-slate, as granitic 

 gneiss does to that which is distinctly stratified. Now, in 

 Scotland, Norway, and those countries where granite and 

 gneiss are characterised by containing hornblende, it is a 

 common occurrence for gneiss and hornblende-schist to al- 

 ternate together : so far the igneous theory goes hand in 

 hand with nature ; for if clay-slate be converted into horn- 



