1864.] T. STERRY HUNT ON LITHOLOGY. 16^ 



Besides these great trachytic hills, numerous smaller masses of 

 different varieties of trachyte, in the form of dykes and beds, are 

 found along the line of country between Rigaud and Yamaska 

 Mountains. The diorite of the latter is cut into dykes of a white 

 or brownish-gray trachyte, which is often porphyritic, and may be 

 connected the great mass just described. 



Chambly. — At Chambly a mass of porphyritic trachyte is in- 

 truded in the form of a bed among the strata of the Hudson 

 River formation ; and about midway in the Chambly canal a simi- 

 lar trachyte is met with, which contains in drusy cavities, crystals 

 of quartz, calcite, analcime, and chabazite. The base of this 

 rock is of a pale fawn color, and appears at first sight to be 

 micaceous; but on closer examination it is seen to be almost 

 entirely feldspathic. Minute portions of pyrites, and grains 

 of magnetic iron, are rarely met with, and small scales of a 

 dark green micaceous mineral are very sparsely disseminated. The 

 crystals of orthoclase, which are very abundant, are sometimes an 

 inch in length, and one fourth of an inch in thickness : they are 

 more or less modified, and terminated at both ends. They are easily 

 detached from the rock, and are yellowish and opaque on the exte- 

 rior, but the inner portions of the large crystals are transparent 

 and vitreous. The composition of the crystals is given under vil. 

 The paste of this porphyry, when carefully freed from crystals, lost 

 by ignition 2.1 per cent. When pulverized and digested with dilute 

 nitric acid, it effervesced slightly, giving off carbonic acid, together 

 with red fumes, arising in part from the oxydation of the pyrites. 

 The portion thus dissolved equalled carbonate of lime 1.76, car- 

 bonate of magnesia 0.98, peroxyd of iron with a trace of alumina 

 2.12 per cent. The residue, dried at 300° F., gave the result viii. 



TII. VIII. 



Silica 66.15 67.60 



Alumina 19.75 18.30 



Peroxyd of iron 1.40 



Lime 95 .45 



Potash 7.53 5.10 



Soda 5.19 5.85 



Volatile 55 .25 



100.12 99.85 



The paste of this trachyte thus differs but little from the crys- 

 tals in composition. It contains only a slight excess of silica, and 



