02 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 oio, 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- 



