82 
ALASKA GEOLOGY 
nearly vertical and at right angles to the strike of the 
sandstones, appearing to follow the direction of their 
dominant jointing. 
The dike rocks vary widely in petrographic character, 
and in the absence of chemical analyses can not always be 
classified satisfactorily. Certain types of rock were found, 
however, to be persistent, and descriptions of these will 
perhaps give an adequate idea of the intrusive bodies. 
ALKALI-SYENITE-PORPHYRY 
Several dikes of an alkaline hornblendic porphyry were 
found, and as they are in appearance the most striking 
rocks collected, and have proved to be the most interest¬ 
ing petrographically, they will be described first. 
Nos. 78, 98 and 99 are from three similar dikes about six feet in 
width, cutting vertically the lower Stepovak beds near the western 
foot of Chichagof Peak. They are abundantly porphyritic rocks, with 
numerous shining black hornblende crystals in a compact greyish to 
buff-colored groundmass. The hornblende crystals are as much as 
half an inch in length, slender, bounded by planes of prism and pina- 
coids, no, 100, and 010, and showing very perfect prismatic cleavage. 
They may sometimes be seen to surround a core of a lighter green 
mineral which proved to be diopside. 
Rather numerous small irregular cavities occurring in the rock are 
lined with bright green crystals of epidote and sometimes are filled with 
granular quartz. The weathered rock develops a platy parting paral¬ 
lel to the dike walls, and the groundmass becomes soft and crumbly, 
showing cavities filled with fibrous masses of laumontite. 
In thin section the groundmass proved to consist chiefly of minute 
feathery crystals of feldspar showing over considerable areas a rude 
parallelism of their longer dimensions. The feldspar has a lower index 
of refraction than the balsam, and is rather obscurely twinned on the 
albite law, with somewhat wavy extinctions at low angles. These 
characters point to albite or an acid oligoclase. With the feldspar are 
numerous grains and tiny prisms of colorless diopside, abundant mag¬ 
netite grains, and here and there a needle of pale green secondary 
hornblende. In one section an insignificant amount of quartz was 
found in the groundmass, but it is generally lacking. The pheno- 
