Gregory] NORMAL TUFF. 123 
quartz, and again are broken up into dusty iron patches within the 
general lapilli boundary. The few feldspar crystals present are too 
badly altered to calcite to be definitely determined, and little aggre- 
gates of calcite grains and crystal sections are sprinkled through the 
whole groundmass. 
PI. XIV, B, shows the typical tuff for this region. 
Some A^ariations from this typical lapilli-nlled tuff will be here briefly 
mentioned. 
In the road running north-south along the east of Castle Hill the 
rock has much more the appearance of an argillaceous sandstone, and 
its ashy character is only revealed by weathering. The microscope 
3hows quartz grains and fragments of andesitic material in addition to 
the vesicular glass and lapilli. 
A specimen collected near the main outcrop of the normal type, as 
described above, was found by microscopic examination to be composed 
of large glassy fragments mixed with a few feldspar rods. The bub- 
bles in the vesicular igneous fragments are elongated and arranged 
rudely in rows. The whole structure suggests a lava stream which 
bad been blown to fragments while in process of flow. The rock is 
now stained with iron and dotted with chlorite, while the interspaces 
ind vesicles are filled with calcite. 
The material forming the Richardson Hill knob, and also that near 
the silicified tuff on lot 31, contains few lapilli, but much vesicular lava, 
quartz, and feldspar, and in some of its varieties passes into the vol- 
3anic conglomerates which are without fragments of glass. 
On lot 31 there is a coarse variety of ash, loose-textured where 
exposed to the weather, and composed of angular and subangular grains 
)f quartz and various volcanic fragments. The larger fragments are 
so firmly embedded that they break across when the rock is broken. 
A few fossils, mostly corals, are embedded with the rock fragments. 
Che microscope shows all the grains to be angular, some remarkably 
agged, and to consist of quartz in individuals or groups; feldspar, 
mtwinned or with Carlsbad or albite twinning, and occasionally 
nicroperthite intergrowths; light-colored trachytic and andesitic 
Tagments; microcrystalline masses of quartz and feldspar marked by 
oerlitic cracks and representing devitrified glass, and finally lapilli* with 
resides and characteristic outlines. 
These looser-textured fossil-bearing ash beds on the southern slope 
>f Castle Hill could scarcely be distinguished in the hand specimen 
rom the sandstones of the Livingston formation in Montana, 1 and the 
)resence of abundant andesitic fragments is common to the material 
rom both localities. 
iW. II. Weed, Bull. U. s. Geol. Survey No. L05. Weed and Pirsson, Bull. U. S. Geol. Survey No. 
19, pp. -49 et acq. 
