THE CRGWSNEST VOLCANICS. 
17 
Titanite occurs as phenoerysts in characteristic lozenge 
shaped, twinned individuals, up to 0*7 mm., long. 
The groundmass is apparently not completely crystalline. 
About 20 per cent of it consists of well-shaped rods of green aegi- 
rite-augite up to 0*1 mm. long, embedded in a mass of highly 
irregular interlocking feldspar grains, somewhat decomposed. 
Melanite Bearing Trachyte. 
A hand specimen (field number 5) of a breccia of well round- 
ed rock fragments in a tufaceous, green matrix (collected from 
a bedded series of tuffs and breccias near the base of the volcanics 
on the West Branch Southfork river, just above Lost creek) shows 
several pieces of a trachyte containing roughly 40 per cent of tabu- 
lar red orthoclase phenoerysts, up to half an inch long, embed- 
ded in a dense green matrix containing a number of shiny black, 
dodecahedral melanite phenoerysts. This type of trachyte is 
frequently observed as fragments in the breccias, and several 
varieties have been noted. 
These rocks are characterized in thin section by large pheno- 
erysts of orthoclase, (in part sanidine) with smaller onesof segirite- 
augite, garnet, and titanite, in a groundmass which varies in 
different specimens, but is characterized by laths of feldspar, 
doubtless orthoclase, aegirite-augites, and magnetite. 
The orthoclase phenoerysts are usually somewhat altered, 
often show Carlsbad twins, and form up to 40 per cent of the rock. 
The pyroxene is up to 0*5 mm. in size, grass green in colour, 
with a grass green and yellow pleochroism, and occurs as eight- 
sided idiomorphic crystals. Extinction angles C A c of 75 degrees 
have been noted. In some rocks the segirite-augite is altered to 
pseudomorphs of matted green scales, mostly chlorite, but is 
frequently found quite unchanged, 
Melanite is usually under two millimetres in diameter, 
typically idiomorphic and of a yellow to deep brown colour. 
It commonly has beautiful zonal banding, and occasionally 
holds small inclusions of aegirite-augite. It is always fresh. 
The relations of the melanite to the groundmass and to the other 
