T. S. Hunt on Lithology. 93 
system. The principal undulations of these rocks have, like 
those of the Appalachians, a north and south direction; but 
this syenite extend from the central mass, and traverse the sur- 
rounding gneiss and limestone. Numerous dikes of quartzife- 
Tous porphyry intersect both this syenite and the surrounding 
gneiss, and are seen in one case to proceed from a considerable 
nucleus of porphyry, which rises into a smal] mountain; render- 
ing it probable that numerous other porphyry dikes of the re- 
gion radiate in like manner from other nuclei of the same rock, 
noticed a rock which has for its base a compact-petrosilex, or 
intimate mixture of orthoclase and quartz, rendered porphyritic 
at Grenville, where it forms dikes in the syenite of that misono 
se 0 
og her mes v 
abundant; and less frequently small grains of nearly colorless 
translucent quartz. An analysis was made of a characteristic 
Variety of the rock, the base of which was greenish-black, jas- 
Per-like, conchoidal in fracture, and feebly translucent on the 
€dges, with a somewhat waxy lustre. The hardness was nearly 
equal to that of quartz, and the specific gravity 2°62. <A few 
distinct crystals of red orthoclase, and some grains of quartz, 
Were present. The base, freed as much as possible from these, 
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