THE CROWSNEST VOLCANICS. 
29 
erates , and rocks of finer grain than this are termed tuffs. The 
definition is arbitrary, and the dividing line of size may not be 
agreed to by all, but the reader will at least know the general 
textural appearance of the rock from the name applied to it. 
It will be recognized that tuffs as here defined are rocks much 
in appearance like sandstones, while agglomerates, with their 
coarser grain, simulate conglomerates texturally. A rock like 
specimen 39, described below, containing several fragments 
of feldspar up to an inch in diameter, embedded in a fine tu- 
faceous matrix, is here called a tuff, owing to the predominance 
of very fine material. Its analogue in the detrital sediments 
may be taken as a pebbly sandstone, but there are objections to 
the term “pebbly tuff.” 
Orthoclase Tuff. 
Constantly recurring in the exposures of the volcanics are 
rocks in appearance greatly simulating porphyries, and at first 
mistaken for flows in the field. They are alike in containing 
rectangular crystals and broken fragments of orthoclase (some- 
times soda orthoclase and sanidine) up to an inch long, embedded 
in a fine tufaceous matrix, whose clastic character is sometimes 
only certainly determined by the microscope. The resemblance 
between specimen 39, a soda orthoclase tuff, and specimen 35, 
an segirite-augite trachyte, is particularly striking, in colour, 
size, and, to a great extent, shape, of the phenocrysts, and also 
in the colour and texture of the matrix. Specimen 87, a sanidine 
tuff, is also remarkably like a porphyry in the hand specimen, 
and numerous other cases have been observed in the field. 
In the case of the two rocks just mentioned, it is thought 
that the segirite-augite trachyte magma, on explosion gave rise 
to the soda orthoclase tuff. The question may be raised as to 
why the pyroclastic rock is not called segirite-augite tuff, in 
conformity with its parent primary type. In the case of the 
trachyte, segirite-augite is the definitive mineral, distinguishing 
it from other trachytes, while in the case of the tuff, soda ortho- 
clase is the distinctive mineral of the rock. 
