38 THE ROYAL SOCIETY OF CANADA 



dykes masses of porcellophite and aphrodite, a few flakes of brown 

 mica, a few crystals of reddish brown garnet, and occasionally a crystal 

 of tourmaline or zircon. Dykes at the American Chrome pit some- 

 what similar in character were found to contain kaolinite, which is 

 generally regarded as a decomposition product of aluminous silicates 

 such as the feldspars. 



There are then in the dykes with the colerainite certain minerals 

 like mica and garnet which could not have maintained themselves 

 against the solvent action of hot magmas and still others like kaolinite, 

 porcellophite, and aphrodite which are undoubtedly formed under 

 the influence of cold solutions. All the evidence upon the point, there- 

 fore, indicates that colerainite has been formed near the surface of 

 the pegmatite dykes as a result of the interaction between aluminous 

 materials of the dykes and magnesium salts carried down by surface 

 waters. 



The alteration of aluminous silicates, chiefly feldspars, in the 

 dykes has given rise to the formation of hydrous silicates the most 

 clearly defined of which are kaolinite and colerainite. 



The derivation of kaolinite from albite through the agency of 

 carbonated waters has probably taken place according to the follow- 

 ing equation : — 



2(Si308NaAl) + CO2+2H2O = Al2Si209H4 + Na2C03+4SiO 



Albite Kaolinite 



the carbonate of sodium and the silica being removed in solution. 



It is quite within the realm of possibility that some such change 

 as this takes place preliminary to the formation of colerainite, but 

 as far as has been observed kaolinite is absent from those portions of 

 the dykes which hold colerainite and the latter mineral may have been 

 formed directly from the feldspar by the action of magnesia-bearing 

 waters in accordance with the following formula: — 

 2(Si308 NaAl) + CO2 + 4MgO + 5H2O = 



Albite 



2(AlSi08 MgsHs) -f Na2C03 + 4Si02 

 Colerainite 

 and as brucite is of common occurrence in the area there seems to be 

 little to preclude such a reaction as is here indicated. 



The evidences in favour of the hypothesis which is now sug- 

 gested to account for the origin of colerainite may be summarized as 

 follows : — 



1. None of the characteristic products of pneumatolysis has been 

 observed in the dykes carrying colerainite. 



