Acid Pcg)natytc i>i Diabase. — Jaggar. 205 
idiomorphic crystals; the biotite shows idioniorphic hexagonal 
basal sections, m some cases with a beautiful sagenite web. 
The long magnetite crystal groups, resembling single straight 
prisms, mostly on the border of chloritized hornblende, are 
remarkable; they have frequently a length of from 0.75 to i.o 
nim. and suggest the paramorphic resorption borders of the 
hornblende crystals in trachytes and andesytes, but in this 
case they never completely surround a crystal, and also occur 
frequently in long needles between the feldspar laths; in one 
case one of these long ore needles was seen to intersect a large 
apatite crystal. Pyrite occurs, and secondary infiltrations of 
quartz, epidote and idiomorphic calcite occur in irregularly 
bounded masses in the thin section, stringing out into fissure 
fillings among the larger minerals. The feldspars are more 
basic than in the diabase facies, giving in one case symmetrical 
extinction 19° and 12° in the zone normal to M; this would 
make the feldspar near to Abi Ani, an acid labradorite . 
We have thus three facies of this rock in Somerville. 
(i) Augite — biotite — andesine diabase. 
(2) Hornblende — augite — labradorite dioryte (Hobbs). 
(3) Hornblende — biotite — acid labradorite dioryte. 
In addition a drusy quartz-microcline pegmatite is a com- 
mon feature in the Granite street and Pine hill (Medford) lo- 
calities; this occurs in irregular lenticular or vein-like masses 
merging into the normal diabase by gradual transitions. Ap- 
l)roaching one of these veins, the long white plagioclase crys- 
tals of the diabase are seen to acquire a salmon-colored bor- 
der zone of more acid feldspar; gradually the plagioclase gives 
place to microcline and the long laths are replaced by short 
rectangular pink prisms; quartz replaces all the bisilicates 
and in places the rock appears like a granite; in the open 
druses prismatic milky vein quartz overlying short, well- 
formed microcline crystals, with in some places a green amphi- 
bole and consfiderable calcite, stand out on the walls, having 
all the appearance of infiltration products. Webster, in 1825, 
noted the presence of quartz and a granitic feldspar as abnor- 
mal, saying "in some parts of the bed the feldspar predomi- 
nates and has a fine iiesh color; in one place the prisms of feld- 
spar are an inch or more in length, and cross each other in all 
directions, leaving angular spaces * * '•■" which contain. 
