DETAILED DESCRIPTION OF SLIDES. 127 



no hornblende, it might readily be mistaken for a hornblende-andesite. 

 Under the microscope the slide shows little or no hornblende, but an unusual 

 amount of augite, which is present, not only as porphyritical crystals, but as 

 microlites in the groundmass in nearly the same quantity as the feldspar. A 

 majority of the augites are fresh, but many are decomposed to chlorite, which 

 in its turn is largely changed to epidote. The latter may be seen eating its 

 way into the chlorite, as it has been described in the diorites and horn- 

 blende-andesites. Only a single patch of chlorite suggests hornblende, and 

 there is none of that mineral in a fresh condition. The feldspars contain 

 devitritied glass and secondary fluid inclusions. They are much dimmed 

 bv the presence of chlorite and calcite. 



Slide 315. Siitro Tunnel, 1,400 feet from entrance. 



Variety with felt-like groundmass. Tllis is a dark, rCsinOUS-lookiug TOck, wltll 



some large greenish feldspars. The slide shows many fresh augites well 

 crystallized, somewhat dichroitic, and with a tendency to develop only 

 one cleavage. Others have undergone a somewhat peculiar decomposition, 

 the product of which seems to be chlorite, very heavily charged with 

 hydrated ferric oxide. There are few augite microlites in the groundmass, 

 but many in the feldspars. The feldspars are well developed, and a very 

 few only give angles of extinction answeiing to anorthite, in spite of the 

 fact that a considerable number shovv periclinic twinning, and seem to be 

 cut nearly in orthopinacoidal section. A good many such give almost 

 exactly the theoretical maximum angle of extinction of labradorite, and 1 

 incline to the belief, for which there is no substantial proof, that they really 

 belong to that species, and that consequently both of the more basic feld- 

 spars are present. There is much magnetite and many dark apatites. The 

 groundmass is a felt-like aggregation of tiny microlites, between which 

 there is certainly a small amount of glass. 



Slide 481. Between summit of Mount Kate and Occidental Giade, near point 5039. 



Glassy variety. — Tlils is a gray glassy-lookiug rock, unlike the ordinary 

 augite-andesite. The slide, however, shows it to be decidedly of that 

 species, and to consist essentially of augite and triclinic feldspar, embedded 



