30 
MUSEUM BULLETIN NO. 4. 
Sanidine Tuffs. 
These are usually fine, even-grained, light greyish green to 
dark green rocks, in many cases indistinguishable from sand- 
stone (and even from traps) in the field. Occasionally specks 
of glassy feldspar or melanite are seen, and even of fine-grained 
rock fragments. 
In thin section the rocks are characterized by a large 
amount of sanidine, mostly less than 1 mm. in diameter, and quite 
angular. The matrix is very fine, not clearly resolved by the 
microscope, and probably consists of finely comminuted feldspar, 
pyroxene, analcite, etc. 
Blairmorite — A gglomerate. 
This is another instance of a pyroclastic rock closely simu- 
lating the crystallized representation of its parent magma. 
Forming the matrix in which the large fragments of blairmorite 
(variety A) were found, is a clastic volcanic rock, consisting of 
about 40 per cent of analcite crystals and fragments, wholly 
separated from their groundmass, which are embedded in a 
greenish fine to medium tufaceous matrix composed of small 
fragments of dense greenish rock, with other pieces of white 
orthoclase and many black garnets (melanite). Some portions 
of this agglomerate are strikingly like the blairmorite in appear- 
ance, owing to the size and proportion of the bright red anal- 
cite crystals and fragments. 
An analysis of the blairmorite tuff described by Knight is 
quoted on p. 23. There is, of course, no very exact similarity 
among the various blairmorite pyroclasties in regard to ultimate 
composition. 
Tuffs and agglomerates of various other sorts are found, 
but all are characterized by different association and proportions 
of the rocks or minerals already described. Many of the pyro- 
clastic rocks are largely replaced by calcite, and the various 
stages of replacement are very interesting. 
