20 GEOGRAPHY AND GEOLOGY OF RAMPART REGION. 
If this interpretation of the section is correct the purple, green, and black slates 
and cherts with associated greenstones are probably the oldest rocks in the region 
and may possibly belong to the Silurian, but from the fact that wherever they have 
been found to occur they are intimately related to limestones, which, where deter- 
minable, have been found to be Devonian, it seems best at present to call them all 
Devonian. 
The contact relations of these rocks with the rocks to the north and south of them 
are not clear. On the south the red and black slates with massive greenish quartzite 
form a ridge just west of Beaver Creek between Beaver Creek and the White Moun- 
tains, and outcrops of the quartzite occur sparingly southeast of Beaver Creek, where 
their attitude is nearly vertical. At the northern end of the section the relations are 
different, Fossils determined as Pennsylvanian or Permo-Carboniferous were found 
in a low, outlying ridge overlooking the Flats, and across a narrow valley on the south 
are black slates and cherts at the base of a high ridge, the topmost points of which 
are quartzites containing occasional chert pebbles interbedded with red and black 
slates and some dark limestones. All of these are regarded as Devonian. 
The evidence at hand seems sufficient only to justify the statement that in the 
section from Chatanika River to Yukon Flats there is a large area of closely folded 
rocks, mostly of Devonian age, flanked on the south by highly metamorphosed schists 
and on the north by slates partly similar lithologically to the Devonian slates, but 
containing Carboniferous fossils. Any interpretation of the section on the basis of 
our present knowledge is open to objections. For example, dips were observed in 
the White Mountains which indicate a local anticlinal structure, but the folding has 
been so close that only more detailed studies of the structure supported by more 
than the fragmentary paleontologic evidence at our disposal will afford the basis for 
a reasonable approximation to the truth. The present hypothetical interpretation 
of the section is regarded as a temporary basis only, the first at hand, perhaps, for 
the preliminary description of the material occurring in this area. Whether it shall 
be found sufficiently strong to be given the dignity of a permanent basis in the 
explanation of the structure or must be replaced by one better able to bear the 
accumulating weight of evidence is, for the purposes of this report, immaterial. 
Passing to the Rampart region there is found a variety of rocks like those already 
described. The disturbance which they have undergone has been greater and intru- 
sion by igneous material more active. Folding has been intense, many of the rocks 
have been greatly sheared and in some cases brecciated by the forces to which they 
have been subjected. A section northward from Baker Flats to Rampart is shown 
in PI. I. The strike of the rocks is nearly east and west. The garnetiferous mica- 
schists and marbles of Ruby Creek, considered as pre-Devonian, are followed on the 
north by cherts and greenstones and on the south by rocks including slate, chert, 
sheared chert conglomerate, fine-grained rocks having the same composition as the 
chert conglomerate and also sheared till they have been rendered schistose, massive 
quartzites, and siliceous limestones, in places much brecciated. Here, also, the suc- 
cession seems to be from chert and slates through chert conglomerate to fine slate 
and limestones. 
There are other localities in the Rampart region where a partial succession has 
been observed. The main divide to the east of Lynx Mountain is composed mostly 
of chert flanked by chert conglomerate, and at one locality near the southeastern 
base of Lynx Mountain there are fine exposures showing at the base a conglomerate 
containing sheared chert pebbles several inches in diameter, changing gradually to 
alternating beds of finer material. At a locality not far from this gray slates were 
observed highly folded and cleaved and pitching eastward. These slates contain 
thin beds of quartzite a few inches to a foot or more thick which contain grains of 
chert. 
In Troublesome Valley the succession seems to be the same. A section southward 
