GEOLOGY OF LITTLE ROCKY MOUNTAINS 425 
varied, flamed, or clouded aspect. They are probably similar 
in some respects to the moiré feldspars described by Broégger. 
The mingling is of too fine a nature and the particles too minute 
for the two varieties to be separated and distinguished by optical 
or chemical means. These fine particles, which are believed to 
be of albite formed by a secondary breaking up of the soda 
orthoclase or anorthoclase molecule—are in general oriented 
similarly with the main feldspar, but not always; in the main, 
however, the section extinguishes similarly over the whole field. 
Frequently, also, the phenocryst has a fine outer mantle or skin 
of the same substance. Scattered through the feldspar pheno- 
cryst thus composed, are great quantities of slender laths of 
albite. They peg the large phenocryst through in every direc- 
tion and present no regularity of orientation with it, or with one 
another. They are twinned according to both the Carlsbad and 
albite laws; often the lath is twinned in halves and as the Carls- 
bad halves have a nearly simultaneous extinction with the albite 
twins it is difficult in this case to determine which method is 
present. That the laths are of albite is shown by optical tests 
mentioned above, where both twinnings are present, and is to be 
inferred from the chemical test made for lime. In a few cases 
the phenocrysts contained these inclusions as short, broad sec- 
tions oriented in zonal planes. It is believed that these included 
albites are not secondary, but are of the same age as those in the 
eroundmass, and that their presence shows that the phenocrysts 
containing them are also of the same’age. Thus the phenocrysts 
spreading outward in their growth would include the albite 
microlites already formed, but which had not yet developed the 
stout, thick form, which at present characterizes them in the 
groundmass. 
Summary of petrography.—The study which has been made 
upon these rocks of the Little Rocky Mountains shows them to 
belong in the alkali granite-syenite series. The magma which 
formed them has cooled and crystallized under conditions which 
gave rise to the granite-porphyry rather than the granular type 
of structure. On the whole it has been very free from lime, iron, 
