GEOLOGY OFVLITTLE ROCKY MOUNTAINS 415 
percentage of lime, iron, and magnesia. In spite of the presence 
of the oligoclase the rock clearly belongs in the alkali series ; 
had more iron and magnesia been present we should expect the 
lime would have been partly exhausted by the production of 
augite or hornblende; their absence has forced it into the feld- 
spar. Their absence also explains why with such a comparatively 
high silica per cent. so little quartz is present; it has nearly all 
gone into the production of feldspar, of which the orthoclase 
molecule demands 64.7 per cent., the albite 68.7, while only a 
very small proportion of the anorthite molecule with SiO,—43.2 
is present. 
If we neglect the minute amount of potash present in the 
muscovite and consider all the alkalies and lime as present in 
the form of feldspar molecules, their molecular proportions 
K.O - - - = : = : ; 503 
Na.O - - = = - = S 5 783 
CHON) : : : : : : : 179 
show that they are in round numbers present as follows: Or,,, 
Ab,,, 
ent show it to have approximately the composition Ab, An, and 
and An,. Since the optical properties of the oligoclase pres- 
since albite has not been observed, it is clear that anorthoclase 
must be present to a considerable extent, in the groundmass. 
The lateral ridge at the head of Alder Creek, southeast of 
Sullivan Butte, is formed of a rock that is slightly different 
from that of the main ridge, and which being free from 
quartz, is classed as a syentte porphyry. This is cut by zones or 
leads of decomposed rock, several of which have been pros- 
pected. Near the Hawkeye mine where the rock has undergone 
secular disintegration, the feldspar phenocrysts have occasionally 
weathered out and form a coarse, sandy débris covering the rock 
outcrops on the summit of the ridge. The rock is compact, 
dark, pinkish gray in color, with abundant white phenocrysts of 
orthoclase which are from 1o™ to 20™™ across, small pheno- 
crysts of an opaque white feldspar, and small cavities due to the 
decomposition of some ferro-magnesian mineral are common. 
Under the microscope the rock is seen to consist of large 
