Gregory] HOKNBLENDE-ANDESITE. 173 
hornblende, plagioclase, and orthoclase, together with considerable 
secondary calcite. The feldspars range from '2 millimeters in length 
down to minute microlites. The larger feldspars are commonly con- 
verted to calcite, which indicates their basic character, but also pre- 
vents their accurate determination. Those which could be measured by 
the Michel Levy method proved to be andesine, with formula Ab t An n 
hence more acid than the feldspars of the augite-andesite. They contain 
glass inclusions, are zonally built with an occasional unaltered outer 
border, and are twinned according to the Carlsbad and albite laws, but 
with very irregular intergrowths of the parts. 
Hornblende is the only important ferromagnesian mineral present, 
and occurs, as the feldspars, both as large basal sections and long laths, 
often with good crystal outline, and also as shreds in the groundmass. 
The larger pieces are rarely in a good state of preservation, but occur 
with ragged edges and show resorption phenomena. The crystal is 
eaten into, and part of the interior converted into magnetite with a few 
augite grains. Some crystals have been almost entirely replaced by 
calcite and magnetite, and others are represented by a ghostlike out- 
line of magnetite dust. Commonly the hornblende is now changed to 
a green micaceous material, perhaps a variety of chlorite, with parallel 
extinction and a pleochroism, C = white green, (I thrown green. At 
times the former crystal is striped in the direction of the cleavage 
cracks, with alternating bands of green and white. Some of the crys- 
tals classed as hornblende are so altered that it is impossible to say 
that they may not be augite. 
The groundmass is formed of small, stringy, ragged feldspars, and 
varies in different slides from a trachytic or pilotaxitic type, with 
possibly a little glass, to a type formerly quite glass} 7 and showing 
devitrified areas with incipient micropoikiiitic structure. The feld- 
spar microlites could not be accurately determined, but their average 
extinction indicates a variety as acid as oligoclase andesine, and i f si rict 
nomenclature were to be considered the rock would be classed as 
a trachyte-andesite. 
SOUTHERN MAPLETON ANDESITES. 
These occur in several localities, and are either identical with or 
present only minor variations from the Edmunds Hill and Hobart 
Hill masses. The rock which outcrops in the road 2 miles cast of 
Mapleton Village has the most glassy groundmass of all the andesites, 
and its devitrihed areas have the micropoikiiitic structure the best 
developed. Two outcrops show a type much lighter in color, with 
much secondary and some original quartz, giving the rock a dacitic 
facies. The other sections examined are of the typical augite-andesite or 
hornblende-andesite of this region, and require no detailed description. 
