ORIGIN OF CERTAIN CONTACT ROCKS. 181 



The chlorite rocks described — Group A, Sub-Group (b) — are 

 found at Anderson's Creek, near Beaconsfield, actually within the 

 boundaries of the serpentine and pyroxenite of that district. The 

 particular varieties observed are those of massive habit and complex 

 mineralogical composition — serpentine, clinochlore and pennine 

 being the dominant constituents. To the occurrence of this type 

 within the basic igneous rock boundaries the author would attach 

 special significance.^ 



The remaining groupings are almost invariably found within 

 sedimentary terrains,, but without exception in the immediate 

 vicinity of basic igneous rocks either identical with or closely 

 related to those which have been observed at Anderson's Creek. 

 And the chlorite rocks themselves also occur thus in more than 

 one observed locality together with the other types. 



Looking iirstly at the broader features of occurrence of these 

 contact rocks within the sedimentary boundaries one cannot 

 but be struck with certain outstanding facts : — 



(a) The composition of the sedimentary rocks themselves is 



in no case that which might be expected when the high 

 content of lime and magnesia in the aggregates under 

 discussion is borne in mind. The sediments comprise no 

 limestones nor any other highly calcareous rocks, as far 

 as is yetknown. They are somewhat variable in character, 

 but the variations are only those which commonly exist 

 between the normal non-calcareous slates and sandstones 

 of a great sedimentary group. The contact rocks of 

 different tj^es occur, sometimes side by side, within 

 sediments of the same or different characters. 



(b) The neighbouring masses of basic igneous material are 



at no place more than a few chains distant from the 

 observed developments of the contact rocks. These 

 basic rocks are sometimes in large excess over the ex- 

 posed acidic rocks with which the contact rocks are 

 found, and sometimes are entirely dwarfed by the later 

 acidic massifs. 



(c) The acidic rocks are exposed at the present surface to 



very different extents in the localities from which the 

 contact rocks have been recorded. Thus, the Heems- 

 kirk massif is a large outcrop of plutonic rock occupying 

 a considerable area. At Anderson's Creek there are 

 numerous intrusions of granite with aplitic and pegma- 

 titic affinities within the serpentine, but no one of them 

 is of large size. At North Dundas the place of the more 

 deeply-seated types is taken by the intrusive equiva- 

 lents — granite porphyry and quartz porphyry. 

 {d) The acidic intrusions themselves and the surrounding 

 forks ^\<o bear evidence of having been affected during 



1 It would appear that the occurrence is not an isolated one. Similar surroundings are indicated 

 for the occurrence of these minerals elsewhere. Vide: H. A. Miers, " Mineralogy-," 1902 pp 422 

 495, 499, 505. 



