ORIGIN OF CERTAIN CONTACT ROCKS. 



177 



The several rocks are grouped qualitatively according to their 

 mineralogical composition, and this grouping is in entire harmony 

 with their field associations. Two main groups have thus been 

 established. Between them there are many significant points of 

 similarity, and certain specific minerals are common to both groups. 

 The characteristic feature of all the rocks is the high proportion of 

 lime-bearing and magnesia-bearing minerals. These minerals of 

 which these rocks are composed still await chemical analysis. It 

 is therefore impossible to state the exact proportion of lime and 

 magnesia contained in them. However, the lime-bearing and 

 magnesia-bearing species are here tabulated and their recognised 

 empirical formulae shown. Their occurrence in the main groups 

 and divisions here adopted is shown by the crosses in the table. 



A. — Diopside and Chlorite Rocks. — ^The rocks here grouped to- 

 gether are somewhat complex in mineral composition and vary to 

 a notable degree from point to point. They form, nevertheless, a 

 very characteristic group of which the members can be recognised 

 at first sight in the majority of instances. The different members 

 of the group occur together at several places in the Comstock region, 

 and some of them have been found at Anderson's Creek, near 

 Beaconsfield. The following description of the salient features of 

 these rocks will sei^ve to indicate the mineralogical composition of 

 the rocks under discussion, although, for want of space, the descrip- 

 tion of each type is brief. 



(a). Diopside Rock. — ^This type consists usually of medium" 

 grained to fine-grained crystalline aggregates of white or grey 

 colour. The structure is sometimes columnar, and in some cases 

 the columnar aggregates attain notable dimensions. These colum- 

 nar portions occur in veins, traversing the parts which are of finer 

 grain ; and the columns, which are sometimes divergent, are 



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