550 
ELEMENTS OF GEOLOGY. 
ing truly at right angles to the plane of the dike. Some of 
the dikes send oif branches. These dolerites are cut through 
by intrusive syenite, and this syenite, in its turn, is again 
cut and penetrated by feldspar porphyry, the base of which 
consists of petrosilex, or a mixture of orthoclase and quartz. 
All these trap-rocks appear to be of Laurentian date, as the 
Cambrian and Huronian rocks rest unconformably upon 
them.^ Whether some of the various conformable crystal¬ 
line rocks of the Laurentian series, such as the coarse-grained 
granitoid and porphyritic varieties of gneiss, exhibiting 
scarcely any signs of stratification, and some of the serpen¬ 
tines, may not also be of volcanic origin, is a point very difii- 
cult to determine in a region which has undergone so much 
metamorphic action. 
* Logan, Geology of Canada, 1863. 
