142 The American Geologist. Septemi.or, luoi. 
occurs in good sized allotriomorphic individuals which oc- 
casionally reach a length of over one-quarter of an inch (six 
millimeters). This tendency toward the development of a por- 
plnritic structure is quite marked in some cases. 
Dark patches or segregations of finer grained material rich 
in biotite frequently occur in these rocks. No evidence was 
found for considering them as inclusions rather than segrega- 
tions. As already stated, the dioryte varies considerably in 
grain within short distances and these dark patches appear to 
be merely accumulations of such fine grained material. These 
segregations, which are also found in the Rowlandsville gran- 
ite, do not increase in size or number as the diorite mass is ap- 
proached. They will be considered more at length in another 
place. 
The mineral constituents of this dioryte which can readily 
be distinguished are hornblende, biotite, plagioclase, quartz, 
and a little orthoclase. Some accessory magnetite and apatite 
is usually present, together with a little zircon and sphene 
(titanite). Garnets are not uncommon and epidote and chlorite 
occur as secondary minerals. Mineralogically these diorytes 
containing biotite and quartz maybe classed as tonalytes, though 
they contain a relatively larger proportion of the ferro-magnes- 
ian constituents than the typical tonalyte from Adamello, de- 
scribed by vom Rath,* and are poorer in silica. Through a 
decrease in the hornblende and a corresponding increase in the 
biotite they pass into the biotite-granite or mica-dioryte and by 
a loss of biotite and corresponding increase of hornblende they 
graduate into the quartz-dioryte. Such transition types are of 
common occurrence. 
The feldspar of these quartz-mica-hornblende-diorytes is 
mostly acid labradorite, though some orthoclase is also present. 
StaurO'Scopic measurements on cleavage pieces gave — 5° to 
— 6° on oP (001 ) and — 15° to — 17° on x P a^ (010). With the 
loss of the hornblende and increase in the amount of feldspar 
and quartz the plagioclase grows more acid and in the biotite- 
granite (mica-dioryte) it was found to be oligoclase. The 
feldspar is usually finely striated, due to repeated twinning ac- 
cording to albite law. Occasionally one set of striations is 
crossed by another nearly at right angles, indicating the pres- 
♦Zeit. dei- rteutschen geol. Gesellsch., XVI, 1SG4, p. 249. 
