184 PROCEEDINGS OF SECTION C. 



petent sources of lime and magnesia. Their presence, according 

 to the hypothesis here proposed, provides the solution of the 

 problem. 



There is a constant difference of age between the basic and 

 acidic igneous rocks to which reference has been made in this paper. 

 The acidic types are of invariably later date, and are known to 

 penetrate the basic ones in several localities. 



It is of equal importance to note that the two invasions are 

 coincident at several points (three at least)^ along a well-defined 

 axis of intrusion which had already been affected by much earlier 

 igneous activity. 



The magmatic emanations issuing from the acidic reservoir 

 hearths and ascending towards regions of lower pressure have 

 inevitably in many cases passed through the basic igneous rocks 

 surrounding the acidic types. The relation of topographical 

 development to original structure determines the actual pheno- 

 mena of occurrence at the surface. At Anderson's Creek, erosion 

 has removed the rocks surrounding the basic masses, and we see 

 these in juxtaposition with the granite. In other places we find 

 at the surface the contact rocks with which this paper deals 

 traversing a complex of sedimentary rocks and the acidic and 

 basic igneous masses which have penetrated them. There can be 

 little doubt but that, with deeper erosion in such localities, features 

 of occurrence for the contact rocks closely comparable with those 

 now to be observed at the surface at Anderson's Creek would be 

 visible at Comstock and North Dundas. 



The point which it is desired to make is that the fissures or 

 paths whereby the emanations from the granitic magma have 

 ascended cannot but traverse the basic igneous rocks through 

 some portion of their subterranean course, even where they are 

 now seen at the surface to lie within the boundaries of rocks which 

 have a sedimentary origin. 



It is to the chemical reaction of the emanations from the acidic 

 magma hearths upon the walls of fissures which traverse the basic 

 rocks that the author would ascribe the greater part of the lime and 

 magnesia contents of the contact rocks here described. 



It is true that the contact rocks are found beyond the limits 

 of the basic rocks, but such occurrences appear to the author tO' 

 invalidate the hypothesis in no respect whatever. It is supposed 

 that the magmatic emanations have acquired by a process of 

 chemical assimilation a certain amount of the material of the 

 wall-rocks of their conduits. If this assimilation took place before 

 the issuing material came to rest, the transference of part of the 

 igneous rock matter may have been sufficiently pronounced to carry 

 the dissolved material beyond the limits of the rocks whence it 

 was derived. 



IHeemskirk, North Dundas and Anderson's Creek. 



