1855.] FORBES — FOLIATION OF ROCKS. 183 



show themselves in the same crystalHne form, but have hkewise a 

 tendency to arrange themselves preferably in certain figures. Thus 

 crystals of grossular from Brevig, in Norway, almost always occur as 

 hexagonal rings of single crystals, the interior and surrounding mass 

 being only the matrix. Crystals of augite from the Canary Islands 

 were found to arrange themselves parallel to the long axis of the 

 prism. Many other instances might be cited that are doubtless quite 

 familiar. 



I have now only to notice some points regarding the chemical 

 composition of the foliated rocks, as compared with the other truly 

 sedimentary beds. 



This has by some been considered so different as to preclude the 

 idea that foliated rocks have been formed from rocks similar to the 

 present formations, without supposing that some ingredients must 

 have been added to, or taken away from such. 



Thus the amount of alkali has been supposed much greater. On 

 examining the later analyses by Taylor of the carboniferous rocks. 

 Hunt's analyses of the lower and upper Silurian rocks, and others, 

 it will be seen that when we subtract the organic matter and other 

 volatile substances, as carbonic acid, &c., we have then no greater 

 difference than that usually found in different series of the foliated 

 rocks. In fact, many of these contain but very little alkali. 



Bischoff makes hornblende-schist to contain only 1'45 per cent, of 

 potash and soda together, and the averages of two analyses of mica- 

 slate give 6' 67 per cent, of alkali ; but, considering that the quartzy 

 schist contains so much less, the average will probably not be more 

 than we find in the fossihferous strata. Thus Hunt found in the 

 upper and lower Silurian beds of East Canada 5*05 and 5*59 per cent, 

 of alkali respectively. 



In the clay slates (azoic) of Norway, Kjerulf has found* between 

 5 and 6 per cent., and some of the slates analysed by Bischoff gave 

 up to 8^ per cent. ; but whether fossiliferous or not, is not mentioned. 

 I think, therefore, there is no necessity for supposing with Forch- 

 hammer that the alkali in the foliated rocks is derived from the 

 vapours of potash and soda exhaled from the melted granite. This 

 view is so extremely Plutonic, that I think it ought only to be ac- 

 cepted when we can find no more moderate doctrine. It seems to me 

 more probable, that, if there really was a deficiency of alkali (which 

 I doubt) in the strata before being metamorphosed, or rather foli- 

 ated, in Norway (as Forchhammer especially alludes to the Norwegian 

 rocks), we should rather seek to account for its subsequent presence 

 by the supposition that these strata have been changed whilst sub- 

 merged, and by the infiltration of sea-water and consequent decom- 

 position of the salt by silica. 



This is consistent also with the known upheaval of the land in 

 Norway. 



Keilhau has advanced the idea of a silicification of the limestones 

 as they approach the crystalline rocks. I have analysed several ; 



* Norske Mag. f. Naturvid. vol. viii. 



