1864.] T. STERRY HUNT ON LITHOLOGY. 175 



The feldspars of the above trachytes and phonolite offer some con- 

 siderable variations in their composition, especially in the propor- 

 tions of the alkalies. In ix the proportions of potash and soda are 

 nearly the same as in the trachytes of Brome, Shefford, and 

 Chambly ; and the same is true of xil. These are doubtless to 

 be regarded as varieties of orthoclase with a large amount of soda, 

 while in the feldspar from the phonolite the proportion of soda is 

 very small. In x, on the contrary, the large predominance of 

 soda indicates a composition approaching that of albite. It is 

 further apparent, from a comparison of the feldspars of the other 

 trachytes whose complete analyses are not given, that the propor- 

 tions of the alkalies are liable to considerable variation, even in 

 adjacent and apparently similar dykes. All of the above feldspars 

 are probably to be referred to orthoclase, or to albite ; but these, in 

 the earthy trachytes, have undergone a commencement of decom- 

 position ; which consists in the loss of a portion of silica and alkali, 

 and the combination of water, resulting in a formation of kaolin. 

 An admixture of this substance will explain the increased amount 

 of alumina, the deficiency of silica, and the presence of water in 

 the feldspars of the more earthy of these trachytes. 



These trachytic dykes are not confined to the vicinity of Mont- 

 real. To the southward, on the shores of Lake Champlain, there 

 is found in and about Burlington, Vermont, a vast number of 

 dykes of intrusive rock ; some of which appear to intersect the 

 strata of the Quebec group, and others those of the Trenton group. 

 Some of these are described as being of greenstone ; and others, 

 as a white or yellowish-white feldspathic rock, often porphyritic 

 from the presence of feldspar crystals. The base of a yellowish- 

 gray porphyritic dyke from Shelburne, having a rough fracture, and 

 a specific gravity of 2.60, gave to Prof. G. F. Barker, silica 67.30, 

 alumina and peroxyd of iron 19.10, lime 0.79, magnesia, traces, 

 potash 4.74, soda 6-04, volatile 1.70,= 99.67. It contained a 

 little intermingled quartz ; and the mass resulting from the fusion 

 of the rock with an alkaline carbonate, afforded traces of a sul- 

 phuret. (Geology of Vermont, pages 579-707.) 



Somewhat to the south of Burlington, on the west side of Lake 

 Champlain, and near to Essex, there is a great mass of intrusive 

 rock, found in the slates of the Hudson River formation. As 

 described by Emmons, it is interstratified in an irregular manner 

 among the layers of the unaltered sedimentary rocks, and has a 



