126 CONTRIBUTIONS TO THE GEOLOGY OF MAINE. [bull. 165. 
Microscopic description. — Under the microscope the slide presents 
some interesting features. The general mass is formed of areas com- 
posed of groups of feathery feldspars which branch and assume broom- 
like or bushlike forms. Often the brooms are arranged radially 
and have branches similarly arranged, and in such cases the ordinary 
spherulitic cross is seen between nicols. It seems evident that these 
areas are of devitrified glass, and it is possible that some glass still 
remains. Other areas are made up of feldspar laths with trachytic 
structure and having their interstices filled with quartz grains. 
The most unusual feature about the rock is the presence within the 
trachytic parts of aggregates of grossular garnets in well-developed 
crystals and irregular grains. With the garnets are associated quartz 
grains or calcite, rarely both, and in plain light the areas thus com- 
posed usually present a distinct polyhedral outline, as if some original 
mineral had been replaced. The origin of the garnets is not understood. 
It seems improbable that they should have been infiltrated into the rock 
in their present condition and occupy only particular places, and that of 
all the rocks of the region this tuff bed alone should contain them. They 
may, however, be due to metamorphism of some included fragment, 
as suggested by Osann 1 in the case of the andesites from Hoyazo, 
Cabo de Gata; but in the Maine tuff the original fragment is repre- 
sented by a distinct polyhedral outline and hence could not have been 
an irregular schist pebble, but rather some crystallized mineral. 
A piece of augite was found in the rock, and also a few badly altered 
feldspars. The alteration products are distributed in accordance with 
the materials composing the tuff — chlorite in the glassy areas and cal- 
cite among the trachytic fragments. The latter mineral occurs also in 
patches throughout the rock and as the filling of the cleavage cracks. 
VOLCANIC SANDSTONES AND CONGLOMERATES. 
As more fully explained at the beginning of this chapter, volcanic 
sandstones and conglomerates include all those clastic rocks composed 
principally of fragments of igneous materials. They are usually not 
much waterworn or rounded, and indicate rapid deposition of material 
from a source not far distant. The separation of this class from the 
tuffs described in the last section is purely arbitrary and not indicated 
by field relations; and furthermore, even when examined in thin sec- 
tion the two classes of rock are found to grade into each other. In 
amount of igneous material the volcanic sandstones and conglomerates 
range from those containing practically nothing else to those in which 
half or more of the rock is of quartz and sedimentary fragments. 
i Zeitschr. Deutsch. geol. Gesell., Berlin, Vol. XL, 1888, p. 705. 
