IGNEOUS ROCKS OF THE ALGONKIAN SERIES 351 



augite is much more abundant than the feldspar. It is an allotriomor- 

 phic mineral, red-brown when fresh, but frequently entirely gone over to 

 chlorite. The small amount of olivine originally present in the rock is 

 now altered to serpentine and chlorite. Besides the chlorite, which is 

 the chief alteration product, resulting from the plagioclase as well as from 

 the augite and olivine, much secondary calcite has been derived from 

 the feldspar. Apatite is found and titaniferous magnetite, in grains and 

 definite crystals, is abundant. The medium texture of the diabase is 

 fairly uniform throughout the flow. 



OTHER IGNEOUS ROCKS 



Other igneous rocks of petrographic interest, besides the above, are 

 boulders of augite-andesite, diabase, melanite, phonolite, and tinguaite, 

 which occur east of Livingston range and in the river drift along the 

 North Fork of the Flathead river. Of these only the diabase could be 

 traced to its source. It is found as an intrusive sheet 50 feet in thick- 

 ness along the bed of Trail creek, about 4 miles west of the Great plains 

 in British Columbia, where it had been seen previously by Dawson. 

 Notable amounts of greenish plagioclase, radiating in star-like or more 

 complex aggregates from one or more common central points, are found 

 in it. 



DIABASE AND ANDESITE BOULDERS 



This diabase and pieces of andesite make up a large part of the igne- 

 ous rocks in the North Fork river drift. Much more rarely phonolite 

 and tinguaite are found. The first of these carries the black titaniferous 

 garnet melanite. The mineral is distinctly recognizable, with orthoclase 

 feldspars, at times nearly three-fourths of an inch long, in the hand 

 specimen. The microscope reveals a holocrystalline porphyritic struc- 

 ture. Nephelite, which amounts to perhaps 25 per cent of the rock, is 

 seen to be responsible for the greasy appearance in the hand specimen. 

 With it in the groundmass twinned orthoclase is found in a second gen- 

 eration. Of unusual interest as being abnormal are the phenocrysts of 

 apple-green hornblende, recognizable by the cleavage pattern on basal 

 sections, which make up about 10 per cent of the rock. Orthoclase 

 occurs, with the abundant large crystals of garnet, and almost as com- 

 monly observed six-sided pieces of nosean. 



TINGUAITE BOULDERS 



The tinguaite has likewise a porphyritic texture. The orthoclase 

 feldspars are so abundant as to make up 40 per cent of the rock. They 



