Geology of St. Helens Island. 65 



color, and fine grained. The lustre is dull, and the rock 

 breaks with an irregular conchoidal fracture. Under the 

 microscope the following minerals were recognized, plagio- 

 clase feldspar, hornblende, pyroxene, pyrite, magnetite, 

 chlorite and calcite. The structure is typical panidio- 

 morphic. The plagioclase is fresh and occurs in numerous 

 slender lath-shaped individuals twinned according to the 

 albite law. The hornblende is abundant but subordinate 

 to the feldspar, and occurs in long slender phenocrysts. 

 It is brown in color and the pleochroism is strong, ranging 

 from very pale to daik brown, the absorption being 

 C>h>a. The cleavage is good and the maximum extinc- 

 tion along co P co is 12°. The phenocrysts are greatly 

 altered, the product being a pale green almost isotropic 

 chlorite, and magnetite dust. A pale yellow pyroxene is 

 subordinate in amount to the hornblende, and is much 

 altered to chlorite and calcite. Iron ore is present as 

 numerous small grains of magnetite and pyrite scattered 

 throughout the section. Whatever the original base was, 

 it is now represented by a fine calcite-chlorite aggregate. 



The rock is a hornblende lamprophyre, and is allied to 

 the Camptonites. 



As a type of the non-feldspathic class, a specimen was 

 chosen from the faulted dyke which cuts the inclusion of 

 Lower Helderberg limestone mentioned as occurring in 

 the breccia. 



Macroscopically the rock is basaltic in appearance, and 

 is both porphyritic and amygdaloidal. The amygdules 

 are irregular or rounded in form, and contain both calcite 

 and analcite, the former mineral usually occupying the 

 centre of the cavity. Microscopically the rock is com- 

 posed of pyroxene, hornblende, analcite, calcite, apatite, 

 and magnetite. 



The pyroxene is comparatively fresh and occurs in 

 small slender oblong phenocrysts with rounded termina- 

 tions. These lying in all directions interpenetrate to 



