PEOPYLITES OF THE FORTIETH PAKALLEL. 141 



after augite, and a nviiuber of patches of chlorite, which seem referable with 

 some probability to hornblendic forms. The feldspars are triclinic and very 

 closely striated. None of the angles of extinction which I observed exceeded 

 the oligoclase limits. The slide contains very little epidote, but the feldspars 

 and groundmass ai-e clouded with calcite and limonite. While no satisfac- 

 tory determination can be made of this specimen, it seems to answer best 

 to a micaceous hornblende-andesite. 



Exploration of the Fortieth Parallel. Slide No. 226, specinieu No. 21,604. Hills east 

 of Golcoiula Station. 



Macroscopically this rock is of a greenish-gray color tinged with yel- 

 low, and shows porphyritical crystals of mica, hornblende, and impellucid 

 feldspars. Under the microscope it is apparent that the feldspars are rend- 

 ered almost opaque by excessively fine grains of what is seemingly calcite. 

 Some of them are triclinic, others appear to me to be orthoclase, but which 

 are in the majority it is impossible to say. The rock contains quartz in 

 which there are numerous fluid inclusions, some of them containing carbonic 

 acid. The quartz also carries unquestionable glass inclusions of good size, 

 in which devitrification has proceeded only so far that between crossed Nicols 

 one or two bright points appear on the jet-black ground of the isotropic 

 substance. One of these is accompanied by the short cracks in the quartz, 

 which have often been observed, and which so beautifully illustrate the 

 elasticity of silica. One of the numerous apatites, too, contains a glass 

 inclusion hung like a drop on an inclosed microlite which is probably 

 also apatite. The hornblende, and even the mica are wholly replaced by 

 decomposition products, largely oxides of iron. I could detect no trace of 

 augite. The groundmass contains some particles of epidote and chlorite. 

 It is nearly impossible to determine a rock so thoroughly decomposed with- 

 out a study of its occurrence. If the feldspar is triclinic it must be a dacite, 

 for the glass precludes the supposition that it is a diorite. The absence of 

 augite and of well-developed feldspar microlites, the appearance of the 

 orthoclase like larger feldspars, the abundance of fluid inclusions, and the 

 general air of the rock, seem to put dacite almost out of the question. Sim- 

 ilar arguments hold against its determination as rhyolite, and but for the 



