riuNDLE.] GEOLOGIC SKETCH, 37 
facies, and some dikes of it show clearly the characteristic effects of 
dynamic metamorphism. 
GABBRO. 
The most typical occurrence of gabbro is in the ridge that begins 
a short distance above the mouth of Flume Creek, on the north side 
of Seventymile Creek. As far as observed, this ridge is composed of 
gabbro and serpentine cut by diabasic dikes. The rough ridges and 
precipitous peaks in which these rocks express themselves make this 
region one of wild beauty. The gabbro is a dark, greenish-gray, 
evenly granular rock, composed of irregular, equidimensional grains 
of plagioclase and monoclinic pyroxene, with an average diameter 
of from 2 to 3 millimeters. Coarser varieties occur, and sometimes 
there is a parallel arrangement of the minerals. The distribution of 
this rock has not been determined. There is much serpentine and 
many dikes of diabase in the same area. 
DIABASE AND BASALT. 
Isolated diabasic dikes are common throughout the Fortymile and 
Birch Creek regions, but the most important occurrences are in the 
"Rampart formation, which is made more prominent by the presence 
of these rocks than by the less conspicuous sedimentaries. The rocks, 
which probably comprised originally a complex of basaltic flows, 
dikes, and tuffs, are now greatly altered and contain a large propor- 
tion of serpentine. A fresh olivine basalt occurs sparingly. It is 
best developed on Chicken Creek, where it probably occurs as a flow. 
A small dike was observed on Confederate Creek, and on North Fork 
of Fortymile, a short distance above the " Kink," two dikes occur. 
SUMMARY. 
The region is a complex one, composed mostly of metamorphic 
rocks. The oldest rocks are believed to be the granites and gneisses 
of the southeastern portion, the contacts of which with later rocks 
are obscured by dynamic metamorphism and by the presence of simi- 
lar rocks of intrusive origin. These are succeeded by a closely folded 
metamorphic complex, locally separable into the Birch Creek and 
Fortymile formations. The Rampart (Devonian) rocks, which un- 
conformably overlie the others, contain a large proportion of vol- 
canic material, and, like them, have been closely folded. Foldings, 
intrusions, the formation of quartz veins, and mineralization have 
probably taken place at several periods. The material of the Kenai 
was deposited on these older rocks, afterwards folded with all that 
had gone before, and still later worn with them by erosion to an 
undulating, more or less level, surface. Recent elevations and de- 
pressions have been many and have left their traces on the terrace* I 
slopes of many valleys and in accumulations of deposits, like those of 
