ORIGIN OF CERTAIN CONTACT ROCKS. 183 



of lenses and veins within this metamorphic aureole to the north 

 of Trial Harbovu". But they are not limited by the narrow zone 

 in which the rocks of more normal t3'pe occur. The rocks of all 

 the groupings mentioned in this paper extend far beyond the 

 immediate contact zones into the rocks which surround the acidic 

 igneous masses. 



Their vein-like habit is accentuated by the fact that the several 

 component minerals are often to be seen arranged in well-defined 

 bands, in precisely the manner in which the constituents of mineral 

 veins so frequently occur. 



Even more significant are the occurrences of metallic minerals 

 in the contact rocks of both groups at Comstock, Colebrook and 

 North Dvmdas. When the ore-bodies of both the Comstock and 

 Colebrook districts are examined, it is very noticeable that there 

 are all degrees of inter-association of the metallic ores and the 

 non-metallic minerals rich in lime and magnesia. The association 

 is that of synchronously deposited components of one aggregate, 

 and not that of the accidental admixture of the ingredients of 

 two or more phases or epochs of deposition. 



IV. — Hypothesis with Regard to Origin. 



Since the form and surroundings of the various rock-types 

 described appear not to be consonant with a derivation from 

 calcareous sediments by the operation of magmatic emanations 

 from the acidic reservoirs upon them, we must look elsewhere for 

 the source of the very considerable lime and magnesia contents 

 of the rocks. 



It has been pointed out that the acidic magma of the Devonian 

 period of intmsion is, wherever developed in western Tasmania, 

 especially characterised by a content of mineralising agents. To 

 the agency of these mineralisers, when they have been freed from 

 the magmatic hearths, we must certainly refer the genesis of these 

 contact rocks. ^ A Devonian granitic invasion has been claimed 

 by the author to affect a zone which extends certainly from Heems- 

 kirk to Middlesex, and possibly even to Anderson's Creek. Along 

 this zone all the rocks here shortly described are situated. 



But the granite cannot reasonably be supposed to have pro- 

 vided the calcareous and magnesic content of these contact rocks. 

 It is true that soda-lime felspars are present in the granite ; but it 

 is also true that there are only the most sparing traces of lime- 

 bearing minerals in the veins which are contained within the 

 granite borders. Fluorite is undoubtedly rare within these 

 boundaries. 



Turning to the other facts known with respect to the occur- 

 rence of the contact rocks, we are confronted with the constant 

 recurrence of the basic igneous rocks, which are certainly com 



1 This suggestion has long since been made for the limurite grouping. I'ide: W. H. Twelvetrees 

 " On the Nomenclature and Classification of Igneous Rocks in Tasmania." Proc. A.A.A.S., Hobart, 

 1902, p. 305. 



