GREGORY.] DIABASE. I7. r ) 
tains a few feldspar threads, like the body of the rock. Sometimes, 
instead of one calcite-filled vesicle, the same space will be occupied by 
a group of them, or the concave inner border of the large one may 
indicate its formation from several smaller ones. There are some 
glassy oval areas with vesicles visible only under the highest powers. 
All these variations are, doubtless, caused by the fact that different 
sections of similar vesicular areas are exposed in the preparation of 
the slide. The only feldspar phenocryst seen in the sections is rounded 
in outline, has albite and pericline twinning, and is badly altered to 
calcite. Its extinction angle indicates albite or andesine, and, from the 
fact that phenocrysts are usually more basic than the components of 
the groundmass, it is referred to andesine. No ferromagnesian mineral 
is present, but the numerous patches of chlorite and the occurrence of 
augite in similar rock in the immediate neighborhood point to the 
former presence of pyroxene. Besides chlorite, there are present, as 
secondary products, calcite, a few epidote grains, and abundant iron 
ore. One slide is sprinkled full of stringy black iron ore in long 
threads or lines of partly connected dots, which are arranged to form 
barbed arrows or a network of fibers that cross at angles of 60° and 
90°, thus imitating the sagenite structure of rutile. 
The groundmass is of long, stringy, narrow, frayed-out microlites 
of feldspar with trachytic structure. Measurements of many laths 
gave practically a parallel extinction, thus indicating oligoclase. 
Expansion structure is developed where the vesicular areas are large 
enough to affect the orientation of the minute laths constituting the 
main body of the rock. 
ANDESITE ASH BEDS. 
Beds of volcanic ash of an. andesite character are represented in 
the region covered by this report. They are particularly abundant 
about Castle Hill, and are discussed in the chapter on clastic rocks. 
DIABASE. 
AROOSTOOK FALLS DIABASE. 
The diabase from Aroostook Falls is a dark-gray rock with charac- 
teristic lamprophyric appearance, quite dense and uniform in texture 
to the unaided eye, with the exception of numerous grains of pyrite 
and a few greasy-looking feldspars which give it a slight porphyritic 
facies. It weathers to an iron-rust color, but the face of the dike 
next the limestones often shows a jet-black slickensided surface, as if 
some carbonaceous matter had been produced by the intrusion. 
Under the microscope the minerals found in this section are apatite, 
iron ore (mostly pyrite, which is abundant), pyroxene, and plagioclase, 
with chlorite as an alteration product. The larger feldspars ( 1 milli- 
meter in length) form crystals whose basic character is suggested by 
their alteration to calcite, but which are too much decomposed to 
