Vol. 4] 



Lawson. — The Robinson Mining District. 



311 



the porphyry rests, that is, as a dyke cutting the limestone but 

 not the porphyry. If the monzonite porphyry is thus to lie re- 

 garded as directly and genetically connected with the monzonite 

 of Weary Flat as a dyke facies, it is remarkable that the pheno- 

 crysts of the small intrusive masses (dykes) are as a rule many 

 times larger than the crystals of the same mineral in the larger 

 batholithic mass of which the dykes are apophyses. In support 

 of the correlation of these dykes with the "Weary Flat monzonite, 

 the further fact may be mentioned that the contact phenomena 

 of both rocks against the limestones is closely the same, particu- 

 larly in the development of large bodies of garnet rock. On 

 the other hand it is somewhat remarkable that none of these 

 dykes have been observed anywhere on the immediate periphery, 

 or, indeed, in the vicinity, of the main mass of monzonite at 

 Weary Flat. In view of the absence of direct evidence of con- 

 nection between the two rocks, it must be confessed that the 

 question of their relationship remains as yet unsettled, but the 

 view that the dykes are apophyses of the batholith seems fairly 

 probable. 



THE COPPER-BEARING PORPHYRY. 



The term porphyry is here used in the popular sense. In 

 mining communities the term is applied very freely to the lighter- 

 colored volcanic and intrusive rocks in much the same way that 

 the expression trap is used for the darker rocks of basaltic 

 character. It is very rarely applied to rocks of plutonic habit. 

 It is particularly used for intrusive and volcanic rocks with 

 which ores are directly or indirectly associated. The usage is 

 not definitive and affords little or no intimation as to the place 

 of the rock so named in any system of classification. To the 

 petrographer the term is simply descriptive of the presence of 

 a porphyritic structure which is common to many different kinds 

 of rocks ; but in the popular usage even this is not essential 



In the present case the popular term is the more readily 

 adopted as a preliminary designation from the fact that owing 

 to its very thorough decomposition the rock we are here con- 

 cerned with is difficult to classify. It is a light yellowish or 

 whitish feldspathic rock with a prevalent porphyritic structure 



