1864.] T. STERRT HUNT ON LITHOLOGY. 165 



rally distinctly crystalline and cleavable, and so far as observed, is 

 not associated with any triclinic feldspar. The hornblende is ap- 

 parently subject to decomposition, becoming soft, earthy, and ferru- 

 ginous in its aspect, while the feldspar retains its brilliancy. The 

 partial analysis of such a specimen of the syenite gave only 0.56 

 of lime, and traces of magnesia, with 3.75 per cent, of peroxyd of 

 iron, and of alkalies, potash 4.43, soda 4.35. This large proportion 

 of soda is also to be remarked in the orthophyre just described, and 

 in the red orthoclase-gneiss of this region, a portion of which gave- 

 3.86 per cent of potash and 3.70 of soda ; while the red orthoclase 

 from the rocks of this Laurentian series, named perthite by Dr. 

 Thompson, gives in like manner 6.37 of potash to 5.56 of soda. 

 A nearly pure potash-orthoclase, generally white in color, is how- 

 ever found in some of the stratified Laurentian rocks. (Geology 

 of Canada, page 474.) 



This syenite of Grenville has in some portions undergone a. 

 peculiar decomposition, which has reduced it to a soft greenish 

 matter having the aspect of serpentine, or rather of pyrallolite. This 

 change has been remarked only in the vicinity of some remarkable 

 veins of chert which are here found cutting the syenite, and as de- 

 scribed by Sir W. E. Logan, is more or less complete for a distance 

 of two hundred yards on each side of them. In specimens of this 

 altered rock, the quartz remains unchanged ; while the feldspar,, 

 still preserving its cleavages, has a hardness no greater than car- 

 bonate of lime. It is somewhat unctuous to the touch, with a 

 feeble waxy lustre, and its color is occasionally reddish, but more 

 often of a pale green. Such a specimen was selected for analysis 

 and gave of silica 80.65, alumina 12.60, lime 0.60, soda and a 

 little potash 2.65, volatile 2.10, magnesia and oxydof iron, traces ; 

 = 98.60. From this result it appears that the feldspar of the 

 syenite has lost nearly two thirds of its alkali ; the iron and other 

 bases having also for the most part disappeared. This removal of 

 the protoxyd bases would appear from the character of the result- 

 ing mineral to be different from that which takes place during 

 the kaolinization of feldspar. The nature of the process requires 

 further investigation, but it was not improbably connected with 

 the deposition of the adjacent chert or hornstone. This substance, 

 according to SirW. E. Logan, forms two large veins which cut the 

 syenite vertically, and have a breadth of from four to seven feet. 

 It is generally arranged in bands or layers parallel to the walls of 



