29 



VI. — Contact Phenomena about the Borders of 

 the Granite Batholiths. 



About the borders of the various areas of granite and 

 granite-gneiss, contact action is pronounced and often very 

 striking. If the invaded rock be amphibolite, fragments 

 torn from it are found scattered about in the gneiss, 

 giving rise to inclusions presenting the various characters 

 already described. 



When the granite invades bodies of limestone, on the 

 other hand, the phenomena resulting from the intrusion 

 are more varied. The invading rock metamorphoses the 

 limestone, and the products of alteration may be divided 

 into three classes: — 



(i) The alteration of the limestone into masses of 

 granular, green pyroxene-rock, usually containing 

 scapolite, or into a rock consisting of a fine- 

 grained aggregate of scales of a dark brown mica. 



(2) Intense alteration of the limestone along the im- 

 mediate contact, into a pyroxene-gneiss or am- 

 phibolite. 



(3) In addition to these alteration products, in certain 

 cases the granite dissolves or digests the invaded 

 rock, having altered it in one or other of the 

 manners above mentioned. 



The alteration products of the first class may be con- 

 sidered as due to the heated waters or vapours given off 

 by the cooling magma, that is, to be of pneumatolytic 

 origin; while the alteration products of the second class 

 result from the more immediate action of the molten 

 magma itself. The products of these two classes of alter- 

 ation have much in common, however, and naturally pass 

 one into the other. 



The evidence of the alterations of the second class, 

 whereby the limestone is converted into amphibolite, is 

 briefly as follows: — The sedimentary series, consisting 

 chiefly of limestones interstratified with amphibolites, the 

 former making up about one-half of the volume of the 

 whole, is invaded by the granite batholiths, torn to pieces 

 and scattered as fragments through the invading rock. 

 These fragments are all composed of amphibolite, and 

 none consisting of limestone can be found. The persistence 

 of this phenomenon throughout the whole area suggests an 

 alteration of limestone to amphibolite. 



