18 
MUSEUM BULLETIN NO. 4. 
phenocrysts, and its manner of occurrence in several types of 
rocks leaves no reason to doubt its primary origin. 
Titanite is a constantly occurring mineral of the trachytes, 
in fresh, twinned diamond-shaped and rectangular sections up to 
a millimetre long. 
In one fragment a few small phenocrysts of colourless augite 
were noted. The groundmass of these rocks nearly always has 
a highly developed flow structure, sometimes in complicated 
whorls, more often in parallel and sub-parallel arrangements of 
the minerals. Sometimes the matrix is only partially or very 
minutely crystallized. No wholly glassy groundmass was noted. 
A second generation of orthoclase laths is the dominant mineral, 
with more or less segirite-augite, often altered to chlorite, and 
occasional magnetite. 
Many trachyte fragments seen only in thin section, contain 
no phenocrysts, and may or may not represent non-porphyritic 
varieties. They exhibit several varieties of fine-grained flow 
structures. As no large fragments of non-porphyritic rocks have 
been discovered, it is probable that these pieces are from the 
matrix of a porphyry. 
Ldtt'bt&m 
This rock (field number 25) is represented by a hand speci- 
men enclosing a portion of a rounded fragment about the size of 
a man’s fist, in a green, tufaceous matrix. It contains pink 
tabular feldspar phenocrysts, occasionally up to an inch long, 
averaging about A of an inch, forming about 40 per cent of 
the rock, in a dull green matrix containing small black melanite 
phenocrysts. 
Under the microscope the phenocrysts are seen to be ortho- 
clase and andesine, with smaller titanite and melanite, in a 
very fine-grained groundmass. 
The orthoclase phenocrysts vary from 3 to 10 mm. in the 
section, and are considerably altered. The andesine phenocrysts 
are up to 3 mm, in size, also altered, and show albite twinning 
with narrow lamellae, as well as Carlsbad twins. Extinction 
measurements indicate a value of AbesAn^ for this andesine. 
Melanite and titanite occur sparingly, the former up to 1 mm. 
in size, the latter up to 0*3 mm. 
