98 T. S. Hunt on Lithology. 
Besides these great trachytic hills, numerous smaller masses 
of different varieties of trachyte, in the form of dikes and beds, 
are found along the line of country berwann Rigaud and Ya- 
maska mountains. ‘he diorite of the latter is cut into dikes of 
a white or brownish-gray trachyte, which is often pdorphyritie, 
and may be connected with 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 on the Chambly canal a 
similar trachyte is met with, which contains in drusy cavities, 
erystals of quartz, riers analcime, and chabazite. ‘The base of 
this rock is of a aoe wn color, and appears at the first sight to 
be micaceous; but “aliiier examination it is seen to be almost 
sete feldspathic. Minute portions of pyrites, and grains of 
etic iron are rarely met with, and small scales of a Bim 
oteon micaceous rice are very spar selye disseminated. 
crystals of orthoclase, which are very abundant, are sroetceald 
an ie in length, and one-fourth of an inch in thickness; ao 
or less modified, and terminated at bot 
are sity detached from the rock, and are yellowish and opaque 
on the exterior, vie the inner portions of the large crystals are 
transparent and vitreous. The composition of the crystals is 
given under VII. The paste of this porphyry, when carefully 
freed from crystals, lost by ignition 2°1 per cent. When pulver- 
ized and digested with dilute nitric acid, it eftervesced slightly, 
giving off carbonic acid, eed with red fumes, arising in part 
"from the oxydation of the pyrites. The portion thus dissolved , 
equalled carbonate of lime 1° 76 6, carbonate of magnesia 0°98, 
roxyd of iron with a trace of alumina 2°12 per cent. The 
residue dried at 300° F. gave the result viit. 
Vil. VIII. 
deat REM Gd cai ee ee i 
Alumina, - - - - - - - . 19°75 18°30 
Peroxyd of iron, - - < = : ; : AS: 1-40 
Lime, - - - « = aa é Ms 5 45 
Bobnad he ae nae ae ee 758 «B10 
Soda, ee 519 = B85 
Volatile, -) e004). Si al ee ee “BS 
100-12 99°86 
The pee of oe trachyte thus differs but little from the 
tals in com It contains only a slight excess of lie 
and seems to s ae up of lamellee of SUpanit t, mingled wae 
small portions of carbonates af lime and mag 4A part 
composition, ves rise to the rust red color of the ‘weathered 
bas a of ioe trachyte, 
mtreal.—The island of Monies! offers a great variety of 
: rocks, which traverse both the Lower Silurian strata, 
ent of Sehiete Royal. ‘Some of these dikes are 
