90 ALASKAN MINERAL RESOURCES IN 1905. 
DIKE ROCKS. 
There are numerous dikes of diabase throughout a large part of the Matanuska Valley 
They are more frequent and larger toward the upper end of the valley and are also mon 
numerous wherever folding and faulting is strongest. The coal-bearing rocks at mam 
places are cut by numerous small dikes and sills of diabase, which, wherever it has com* 
into close proximity to the coal, has altered it to a dense, hard coke. Dikes of this genera 
character are common throughout the greater part of the coal field and always have this 
effect on the coal. 
A large intrusive mass of porphyritic rock occupies the mountain north of Kings Creel \ 
and immediately west of the trail. At the upper bridge over Kings Creek, 3 miles above | 
the locality just mentioned, is a sheet of porphyritic rock apparently interbedded in the 
Tertiary coal-bearing rocks. The thickness is at least 50 feet. 
STRUCTURE. 
The Matanuska Valley is a zone of soft Tertiary shales and sandstones, with some com 
glomerates and igneous rocks, included within two masses of plutonic rocks associated with i 
Mesozoic sediments. The Tertiary belt is from 7 to 8 miles wide and has fairly straight 
and parallel boundaries which form the topographic limits of the greater valley within which 
the Matanuska flows. 
The Tertiary rocks are involved in a system of combined folds and faults. The folds are 
open Mini the faults often cut the axes of the folds. The general strike is parallel to the< 
course of the Matanuska, being V (»()° E. below Chiekaloon Creek and N. 75° to 90° W. 
above Chiekaloon Creek. There are, apparently, subordinate folds parallel to the course of 
Kings, Eska, and Tsadako creeks. 
The Mesozoic anthracite-bearing rocks east of Boulder Creek are more closely folded than 
the Tertiary rocks of the center- of the valley. They are separated from the latter by a 
fault or system of faults parallel to the axes of folding. 
COAL. 
AIM. AL DISTRIBUTION. 
Coal outcrops have been seen by the writer on Tsadaka, Kska, Kings, and its tributaries 
Chiekaloon, and Coal creeks, and on the small creeks heading in the Talkeetna Mountains 
between Boulder and Hicks creeks, as well as in the banks of Matanuska River, about! 
miles above the mouth of Chiekaloon ('reek. (See fig. 5.) They have also been reported 
from Boulder, Hicks, and Caribou creeks, from a creek on the south side of the Matanuska, 
i) miles above Coal Creek, and from Little Sushitna River. 
The extent of the area underlain by coal is not very definitely known. There are at least 
70 square miles of coal in the valley of the Matanuska and its tributaries from Tsadaka 
Creek to Hicks Creek, inclusive. This is a conservative estimate, based on outcrops actually 
known to the writer. It is possible that there is a larger area than this, but it seems cer- 
tain from present knowledge that the coal area in the region indicated above can not in 
any case exceed 300 square miles, except by further extensions of this field or neighboring 
fields outside the region visited by the writer, which might increase the area to limits which 
we have no means at present of knowing. 
POSITION AM) SECTIONS OF THE COAL. 
There are at least two distinct kinds of coal in this region, occurring at two widely sepa- 
rated geologic horizons. One is the anthracite coal, which is Mesozoic, and the other 
includes various grades of bituminous coal, which are of Tertiary age. 
ANTHRACITE. 
The Mesozoic coal, as stated above, is apparently all anthracite. It was seen by the 
writer only along the flanks of the Talkeetna Mountains, between Boulder and Hicks creeks. 
This coal has the ordinary physical characteristics of most good coal of this kind. It is 
