422 WEED AND PIRSSON 
The rock is seen in thin section to be almost exactly like 
that of the main crest, except that it contains occasional pheno- 
crysts of quartz and the groundmass is of finer grain. It isa 
typical granite-porphyry with numerous phenocrysts of orthoclase 
and oligoclase, with occasional partly corroded, resorbed quartzes 
all of rather large size in an alkali feldspar, quartzose groundmass. 
This groundmass is excessively fine in grain, almost crypto- 
crystalline, and the contrast between the size of the phenocrysts 
and the fineness of the groundmass is even more striking than in 
that of the main crest. 
Indian Butte, west of Landusky, across whose slopes the 
wagon road has been built, is an extensive body of syentte-porphyry 
breaking through the sedimentary rocks somewhat irregularly. 
The porphyry is a compact, rather dense rock of a pinkish gray 
tint, showing numerous small phenocrysts of white feldspar and 
occasional large crystals of glassy sanidine which are some 10 to 
12™™ across. The rock is stippled with abundant small black 
crystals of hornblende, which vary greatly in size and give the 
rock a general appearance quite different from that prevailing 
throughout the mountains. No quartz is noticed in the hand 
specimen. 
The thin section under the microscope shows large pheno- 
crysts of orthoclase and smaller ones of oligoclase, with well 
crystallized prisms of green hornblende imbedded in an extremely 
fine-granular groundmass of alkali, unstriated feldspar. Apatite, 
titanite, and iron ore are also present. The oligoclase is often 
zonally built with layers ranging from acid andesine to oligoclase, 
this being shown in the optical characters where Carlsbad twins 
occur. The groundmass is peppered through with excessively 
minute shreds of a brownish mineral of strong double refraction, 
which is held to be biotite. The proportion of phenocrysts to 
groundmass is rather large. The hornblende increases in the 
depth of its green coloring toward the periphery. 
Lookout Butte lies a few miles from Landusky, its porphyry 
slopes interrupting the white encircling wall of limestone that 
terminates the mountain slopes on the south. This butte was 
