160 OONTEIBUTTONS TO THE GEOLOGY OF MAINE. [bull. 165. 
Macroscopic description. — When a hand specimen is broken from the 
ledge, attention is at once called to its spongy appearance, which sug- 
gests furnace slag coated with iron rust. This porosity is caused by the 
presence of cavities which average about 1 millimeter and are so num- 
erous as to form nearly half of the rock. Examination of fresher 
material shows that it has an ashy-gray color and dense texture, and that 
the oval cavities which in the surface exposure were empty are here 
filled with black and white calcite. The cleavage cracks are also filled 
with calcite. These vesicles may be so broken as to show in cross sec- 
tion, or they may retain their identity and remain on the surface as 
projecting spheres (see PI. V, B). 
Microscopic description. — When a thin section is studied the vesicles 
present the most prominent appearance. They are \ to 3 millimeters in 
diameter. The smaller ones are usually round, others are oblong, 
while many have a wavy outline formed by several neighboring bub- 
bles whose inside division walls have been broken down. The bound- 
ary of the cavity is marked by a dense substance, probably a glass 
blackened by ferritic material, and the cavities are lined with a layer 
of impure fibrous calcite, which may be a mere film or may fill half 
the space. Three or more concentric layers may form the lining, and 
one or more of them may bulge out to form little bays. The lining is 
often broken, and fragments have become entirely detached and drop in 
toward the center. Sometimes the cavity is half, or even entirely, 
filled with these fragments. Areas are found where the single cavities 
are replaced by a group of minute vesicles which have a definite 
boundary. These cavities are now entirely filled with calcite. The 
feldspar laths of the groundmass are crowded together and arranged 
in a rudely concentric manner about the vesicles. The form and 
structure of these cavities are shown in PL XIV, B : and are explained as 
follows: The water contained in the rock was driven out by the crys- 
tallization of the magma and was free to act mechanically as steam. 
Centers of accumulated steam formed the cavities and have left the 
feldspars in their wreath arrangement. The lining of one or more 
concentric layers of impure iron and lime compounds was then formed. 
Some slight jar then produced the few cracks and broke down the lin- 
ing of the tiny geodes. The rock shows no effects of outside strain 
except these broken vesicles, and the effects just mentioned may be 
due to the action of the solution within the cavity itself. Finally, 
infiltrated calcite has filled all the broken cavities and cracks, and is 
the most prominent mineral seen under the microscope as well as in 
the hand specimen. 
The general groundmass is trachytic, and is composed of quartz 
grains and feldspars with a few large apatites. The feldspars are 
apparently about half plagioclase and half orthoclase, and occur mostly 
in delicate shreds, sometimes thirty times as long as broad. The ends 
