ARCILEAN GROUP. 273 



causes of local variation, which make it necessary to select particular ex- 

 posures for typical comprehensive sections. These last are only to be found 

 in their entirety, without confusion, in localities where neither the cross-trends 

 nor the igneous intrusions have contaminated the record. The best region 

 for such sections is in the west and north, where the latest disturbances 

 have had least effect, and where the rocks have been least commingled with 

 igneous material, but all causes have acted most opportunely ; also in a limited 

 area in the southeast. 



There are no known epochal unconformities within this system. All that 

 can now be safely reported as to the succession of the strata is embodied in 

 the following generalizations: 



1. The base of the Fernandan System is apparently a series of tough, 

 hornblendic schists, possibly eruptive, but perhaps more probably detrital. 

 They seem to be less uniform, less crystalline, more quartzose, and often more 

 slaty than the basic Long Mountain Series of the Burnetan System. These 

 have the appearance of rocks made from the degradation of such material 

 as probably constituted the shore line during their deposition, namely, the 

 mica schists and the basic rocks of the Long Mountain and Bodeville series, 

 respectively; but they may have been largely igneous. 



2. Superimposed upon the basal schists is a thick series of mica schists, 

 so much like some members of the Bodeville Series as to require close obser- 

 vation to distinguish them, but almost always of a clearly discernible detrital 

 origin, which is not to be mistaken even in hand specimens. Exceptions may 

 occur in the vicinity of igneous intrusions. 



3. There is a class of very fine-grained quartz rocks occasionally more or 

 less contaminated with comminuted mica scales. These may have resulted 

 from the wearing down and rearrangement, in quiet water, of the minerals 

 of the acidic rocks of the Lone Grove and Bodeville series. 



4. Magnetite, sometimes with hematite, in a bed usually about 50 feet thick, 

 is a constant accompaniment in a definite position between set 3 and set 5. 



5. Carbonaceous schists overlie the iron ore beds. These are of two well 

 marked varieties: (1) A fine-grained, blue or gray, shaly to foliated, graph- 

 itic schist; (2) a dense, jet black, slaty, rather tough, indurated shale, having 

 at a little distance the appearance of coal or bituminous shale. There is con- 

 siderable variety in the proportion of graphite in the blue-gray beds, but the 

 greater portion is not rich enough for economic purposes. 



6. A great collection of slaty schists, largely chloritic, but not as a rule 

 foliated, lies above the Carbonaceous rocks in most of the sections which 

 have been observed. There may be a doubt whether the coal-black shale 

 may not shade imperceptibly into this set from below, because there appears 

 to be a slight discrepancy between some of the sections given in this respect. 



