ARCH.EAN GROUP. 275 



IRRUPTIVES OF THE FERNANDAN SYSTEM. 



The following generalizations embody all that can be predicated with 

 safety concerning the igneous ejections which occur as associates of the rocks 

 of this period. Although these conclusions must be regarded more in the 

 light of preliminary observations than as strongly intrenched deductions, they 

 may be taken at least as indications of the direction in which future studies 

 may be expected to lead: 



1 . The granites which occur in the characteristic trend of the Fernandan 

 strata, and as ramifying intrusions, are usually different in aspect and text- 

 ure from the fine-grained sandy gneissoid intrusives of the Burnetan System, 

 although they have relations to those rather closer than to the more recent 

 igneous intrusions. 



2. There is an interesting class of white bedded granites which, when 

 present, always seem to accompany the quartzite division of the Iron Moun- 

 tain Series. These beds are probably contemporaneous with the adjacent 

 rocks, although they may be interbedded eruptives. 



3. The real intrusions of this system are, as might be expected, more 

 commonly associated with the schists, where they are somewhat frequent as 

 narrow dikes, and to some extent as branching feeders. But the occurrence 

 of extrusions of somewhat greater dimensions, apparently traversing the whole 

 section, is not improbable, although none of these are known to be of the Pre- 

 Paleozoic age. 



4. The fact that the trend of the Fernandan System has pushed aside the 

 Burnetan trend in places where exposures of the intrusives of both systems 

 occur, seems to imply the plasticity of the older magma as late as the close of 

 the Fernandan period. This partly explains the evidently fluid condition of 

 the intrusives of later date, which may account for the distribution as an- 

 nounced above, and for the composition as given below. 



5. The filling of the dykes is chiefly a red crystalline granular binary 

 granite, well compacted, somewhat less quartzose than the earlier outbursts. 

 The quartz occurs both coarse-grained and as plates, but it is rarely distrib- 

 uted in the very regular manner characteristic of the Burnetan irruptives. 

 The feldspar is chiefly adularia. In short, the general effect is that of a 

 magma which has not been as much compressed as in the case of the prior 

 intrusions. Mica is rarely present, and then usually in much elongated prisms, 

 as if alteration products. 



6. Quartz dikes are less frequent and much less extensive than in the 

 previously formed system. 



7. As in the lower system, there are basic rocks of one set or more which 

 may possibly be interbedded volcanics. Some color is given to this view 



