186 THE CANADIAN NATURALIST. [May 



the constituents of granite. Similarly, the older basic rocks, among 

 the brecciated and intrusive aggregates above described, are the 

 most fusible, while the newer rocks, being most siliceous, are most 

 infusible. At first sight, it is difficult to conceive how a basic and 

 fusible rock could solidify from a melted mass previous to a more 

 siliceous one. But the geological relations of these rocks are such 

 as to aiford the fullest proofs of their igneous origin. It may be 

 urged that such an origin for the oldest and more basic fragments 

 does not appear proved, but their similarity in mineralogical 

 composition with the intrusive members of the aggregate is in 

 favour of such a view. Furthermore, these older fragments 

 shew, in every instance, such an analogy as regards their relation 

 to the intrusive rocks that they cannot be regarded as accidental 

 fragments of other rocks brought from a distance. If their oriain 

 were of this nature, they would not invariably be more basic in 

 composition than the enclosing rock. The fact of their always 

 bearing a certain relation, as regards composition, to the enclosing- 

 rock renders it unlikely that their source is similar to that of 

 boulders in a conglomerate or fragments in a breccia. On the 

 contrary, it would appear more reasonable to regard them as the 

 first products of the solidification of the fluid mass from which 

 the oranites, and other rocks above described, resulted. In 

 pursuing this subject further, it would appear not unreasonable 

 to base some such theory as the following upon the facts above 

 stated. The area now covered by these rocks must at one time 

 have been occupied by a mass of fused silicates. The temperature 

 of this fluid magma and of the surrounding crust has been 

 intensely high, although perhaps very gradually on the decrease, 

 and the extent of the igneously fluid material muit have been 

 such as to render uniformity in its chemical composition an 

 impossibility. Variations in its composition, as well as in the 

 manner of its solidification, may therefore be supposed to have 

 obtained in different parts of the fluid area. According to the 

 proportion of silica and bases present where crystallisation com- 

 menced and progressed, hornblendic rock, mica syenite, or com- 

 paratively basic granite, first assumed the solid form, leaving a 

 part of the fluid or magma beneath or on the outside of it still in a 

 plastic state, but changed in its chemical composition, and rendered 

 more siliceous than the original magma. If the solidification com- 

 menced at a point where the fluid mass was comparatively undis- 

 turbed, the granular varieties of the rocks above described may have 



