GEOLOGY DEVONIAN. 19 
with alternating beds of conglomerate, arkose, and qnartzite, limestones, and largely 
tuffaceous greenstones. These rocks may be roughly divided into at least two groups 
according as cherts, slates, greenstones, and conglomeratic rocks, or massive lime- 
stones with greenstones, slates, and quartzites predominate. It is believed that those 
forming the first division are older and it may become advisable on more detailed 
work to make a separate formation of them. The term " Rampart series " has been 
used by Spurr« in grouping similar rocks occurring in other parts of the Yukon- 
Tanana country. In those areas, however, diabase, tuffs, and green slates are most 
abundantly developed, and carbonaceous slates and limestones, although frequently 
present, are of minor importance. In the areas under consideration, while the green- 
stones are found throughout, black slates, cherts, and massive dark- and light-gray 
limestones are more common and more strikingly characteristic of the formation. A 
section of these rocks is encountered in traveling from Chatanika River northward 
to the southern edge of Yukon Flats, a distance of about 60 miles, and this section, 
although incompletely studied, has afforded some material regarding the structure 
and age of the rocks which compose it. 
The general strike in the southern part of the section is about N. 50° E. and it is 
instructive to note that the direction of structure which is so strongly emphasized 
topographically in the Alaska Range is here repeated. In the northern part of the 
section the strike is more nearly east and west. The rocks are closely folded and in 
most cases their attitude is nearly vertical. There is a distinct symmetrical arrange- 
ment with reference to a northeast-southwest axis, and, so far as our present knowl- 
edge extends, this symmetrical disposition of the rocks seems to be a fact of impor- 
tance in regard to their structure and succession. This repetition is most noticeable 
in the occurrence of limestone. The White Mountain limestones with associated 
greenstones, flanked on the southeast by red and black slates and prominent masses 
of impure quartzite, rind a repetition 15 miles to the north in another limestone belt 
less conspicuous topographically than that of the White Mountains, with a similar 
association of rocks and flanked also on the northwest by similar slates and quartzites. 
The rocks of the middle portion of the section are slates, greenstones, and cherts, 
and, although the relations are not clear, there are two main areas of cherts about 3 
miles apart with black, purple, and greenish slates and some greenstones in the in- 
tervening space. In passing northward from the northern belt of cherts toward the 
northern belt of limestones there are conglomerates containing abundant chert 
pebbles. In passing southward toward the limestones of the White Mountains there 
are also chert conglomerates with pebbles an inch or more in diameter associated 
with finer rocks containing grains of chert and fragments of slate. Black slates are 
also common and thin beds of conglomerate are interbedded with them. The slates 
are closely succeeded by the limestones on either side and, although the direct rela- 
tion of the two was not observed, it is believed that the limestones are younger than 
the rocks above described. The quartzite flanking the limestone on the north, 
which is very similar to other quartzites apparently interbedded with the limestones, 
contains occasional fragments of chert and it seems best for the purposes of this 
report and until further knowledge is available to consider the flanking slates and 
quartzites as partly of the same age as the limestones and partly younger. 
Fossils found in the limestones of the White Mountains have been determined as 
characteristic of early to middle Devonian, and some of them are possibly of 
Silurian age; but the stratigraphic association makes it probable that all the remains 
belong approximately to the same horizon and the determinations point more defi- 
nitely to the. Devonian. No fossils were found* in the limestones of the northern 
belt, but the rocks and their associations are very similar and no reasons were found 
for assigning them to a different position. 
aSpurr, J. E., Eighteenth Ann. Rept. U. S. Geol. Survey, pt. 3, pp. 155-169. 
