108 ALASKAN MINERAL RESOURCES TN 1906. 
The Kenai rocks of the Matanuska Valley differ markedly from 
those of the type section of the Kenai as exposed at Kachemak Bay, 
on Cook Inlet. In this latter locality the sandstones are soft and 
incoherent, the shales are plastic when wet, and the lignite seams 
form the resistant members. The beds lie at low angles in undis- 
turbed attitudes. With this lesser degree of alteration and folding 
may apparently be correlated the fact that the coal of Kachemak 
Bay is of much lower grade than that of the Matanuska Valley. 
The inferior character of the Jurassic coal of the Matanuska region 
compared to that of some of the Tertiary coal of the same province i 
has already been mentioned. The Tertiary rocks have, as a general 
rule, been more highly folded than the strata of older age to the 
north, and it is a fact of some interest that certain Aucella-be&rmg 
sandstones of upper Jurassic age show a lesser degree of consolida- 
tion than the sandstones of the Tertiary. 
7. Pleistocene stream and glacial gravels. — The larger part of this 
formation is made up of the glacial and fluvio-glacial gravels which 
underlie the Copper River plateau to a depth of several hundred 
feet, as exposed in the gorge of upper Matanuska River. They are! 
probably not of economic importance. These gravels should not be J 
confused with the gravels formed in the present streams. 
8. Dikes. — In addition to the bedded volcanics of Jurassic and 
Tertiary ages, dikes and sills of diabase are widely prevalent. They 
reach their greatest development in the region east of Chickaloon 
Creek and along Anthracite Ridge between Boulder and Hicks creeks, 
where they attain a thickness as great as 500 feet. Their texture 
varies from finely granular to coarse ophitic. They cut all the rocks 
of the area, and thick sills of diabase are included between strata of 
Kenai age. It is probable that these sills are the subterranean 
accompaniments of the great outpouring of Tertiary basalts. 
The Tertiary lavas have not been indicated on the map on account 
of the unnecessary detail which they would introduce. They are 
widely distributed and cap the older formations, forming many of 
the peaks and summits of the region. 
STRUCTURE. 
As Martin has indicated, the general structure of the Tertiary coal 
bearing rocks trends northeast and southwest, parallel with the trend I 
of the valley. Open folding parallel with this direction and accom- 
panied by faulting with northeast trend is characteristic. Minor folds 
with axes in varying directions are present, that at the coal openings 
on Chickaloon Creek being an example. Here the axis of an anticline 
and an adjacent syncline trends southeast and northwest. 
Along the northern boundary of the field, between Little Susitna 
River and Eska Creek, the sandstone beds forming the ridge dip 
