174 CONTRIBUTIONS TO THE GEOLOGY OF MAINE. [bull. 16! 
CASTLE HILL ANDESITES. 
Macroscopic description. — The rocks at this place do not have th< 
character of lavas which have formed thick flows, but rather sugges 
the surface of a flow. They are commonly amygdaloidal, or ever 
slightly brecciated and ashy, and associated with them is an abundance 
of true volcanic ash with lapilli. The rock exposed at the southeast bas( 
of the hill is striking in its field appearance. Black, rusty-looking 
spheroidal or elliptical masses of lava, 1 to 2 feet in diameter, firsi 
attract attention as they lie loosely strewn over the surface. Thesolic 
ledge itself is composed of these forms, which have their outlines wel 
displayed by weathering. These sacklike or pillow-shaped'niasses arc 
plainly amygdaloidal on the surface, but usually much denser in the 
interior, and are cemented together by a coarse breccia of rough tabu- 
lar, spheroidal, or irregular jagged fragments of glassy material and 
igneous rock of composition similar to the spheres. In some places, 
noticeably on McDonalds Hill, to the south of Castle Hill proper, thih 
structure assumes the form of a conglomerate of small amygdaloidal 
spheres 6 inches and less in diameter, closely cemented together with 
angular pebbles of andesite and other igneous rocks. Similar structures 
have been described from California, 1 Scotland, 2 and elsewhere. As 
noticed by Geikie, some basic lavas, e. g., the basalt at Acicastello in 
Sicily, 3 assume a remarkable spheroidal or pillow-shaped structure on 
flowing into water or a watery silt, u the spheroids being sometimes 
pressed into shapes like piles of sacks." This may be the explanation 
in the present case. Another interpretation is that the structure 
represents the ropy, rolling surface at the front of a lava flow. On a 
fresh surface the rock is dark bluish gray, uniform in texture, or 
with a rare feldspar phenocryst. While this appears to be the most 
typical of the textures, it is usual to find vesicles now filled with calcite 
and fragments of volcanic debris large enough to constitute a conspic- 
uous feature in the hand specimen. In weathering, the amygdaloidal 
parts go first and leave the more dense igneous and glassy pebbles 
exposed as a very rough surface. PI. VII will give an idea of the 
external appearance of the rougher forms. 
Microscopic description. — Sections were cut from the densest mate- 
rial, and also from that with macroscopic inclusions, and when exam- 
ined with the microscope showed no difference except in size of 
vesicular areas and in method of alteration. Feldspar microliter make 
up the rock, parts of which are developed as areas of vesicular 
lava. The vesicles range from 2 millimeters in diameter to micro- 
scopic dots, and are rudely oval in outline. The large ones are merely 
the larger part of a rounded area of vesicular glassy lava, which con- 
1 Ransome, Bull. Dept. Geol. Univ. California, Vol I, p. 106. Fairbanks, Bull. Pept. Geo], Univ. 
California, Vol. II, p. 40. 
2 Geikie, Ancient Volcanoes of Great Britain. Vol. I, p. 193, 
3 Johnston-Lavis, South Italian Volcanoes, p. 11, 
