122 CONTRIBUTIONS TO THE GEOLOGY OF MAINE. [bull. 165. 
presence of "very coarse conglomerates and trappean rocks," and of 
sandstones, "friable slates," "fine gray grits," and "conglomerates" 
farther to the west. All the varieties mentioned are composed wholly 
or in part of volcanic debris, and range in size of grain from micro- 
crystalline to conglomerates with pebbles an inch or more in diameter. 
The coarser and finer tuffs are interbedded with the more typical sand- 
stones and conglomerates, and give the appearance of dikes or beds of 
hard-baked feldspathic clays. The coarse tuff in particular resembles 
an igneous rock, and microscopic examination is required to determine 
its true character. It is doubtless on this account that the State sur- 
vey overlooked the ash deposits of the region. 
PETROGRAPHY. 
CASTLE HILL TUFFS. 
The same division of the Castle Hill tuffs into normal type and 
silicified type will be made here as in the field description. 
Normal type. — This has its best development on the southeast base 
of Castle Hill, where it forms all the rock exposures seen, with the 
exception of the andesites on lot 31 (see map, fig. 6, p. 120). The 
ash on the west side of the wagon road contains the most numerous 
glassy fragments, as viewed by the unaided eye. It is light lead- 
colored on fresh surfaces and full of lapilli, which appear as white 
areas specked with black dots, made by calcite filling minute cavities. 
The rock breaks into rough fragments, but more easily along lines of 
partly developed cleavage or shearing. Weathering first forms a 
sandy coat very uneven in thickness, and finally the rock changes to a 
crumbling breccia. Specimens picked up from any part of the field 
show evident tuffaceous characters. The microscope shows the rock 
to be more than half composed of lapilli, which are present in two 
forms. These forms are as follows: 
1. Lapilli composed of particles of white dust, which are isotropic, 
very angular in outline, with cusps, crescents, and sherds. In places 
the angles are rounded off and replaced by black dust, and again the* 
characteristic outline of a former sherd shows, which is no longer 
glassy, but filled up with iron dust. They occasionally contain vesicles 
and show gradations in size up to — 
2. Lapilli one-half millimeter in diameter, of white glass, perfectly 
isotropic, and filled with vesicles which may be large or small, separate 
or in coalescent groups. These vesicles are usually round, but occa- 
sionally are drawn out into ellipses and are now filled with secondary 
calcite or rarely with chlorite in radial growths. Boundaries of the 
lapilli are made of concave recesses between projecting cusps formed 
by bursting of steam pores along the edge of the glass fragment. 
These larger lapilli show areas devitrified into feldspar laths with 
