Gregory,! SILICIFIED TUFF. 125 
structure within. These areas arc in part devitrified and composed of 
quartz grains, but they always retain their vesicular structure. In 
other places devitrification has progressed until only dusty, opaque 
spheres remain in a quartz groundmass. Iron pyrite and grains of 
red jasper are quite generally distributed. With the exception of the 
structures described, the rock is siliceous. Sometimes the quartz is 
arranged as a tiny geode; sometimes there are areas of quartz bounded 
by a faint dark outline when viewed in plain light, and everywhere 
throughout the section groups of interlocking grains or individual 
grains, seen only with the highest magnifying powers, are present to 
the exclusion of other minerals and alteration products. The rock is 
considered a good example of a highly acid tuff now completely 
dlicified. 
TUFF IN NORTHEAST PART OF ASHLAND TOWNSHIP. 
The tuff from this locality appears in the coarser variety as a dense 
blue rock, in which are found greenish-gray rounded and angular lapilli 
in eighth of an inch and less in diameter, whose aggregate amount 
comprises more than half the whole specimen. In weathering, the 
ittle spherical lapilli alter to a white porous substance, while the 
entire mass weathers dirty brown, giving the whole surface a peculiar 
spotted appearance, which has given rise to the idea that the struc- 
ture is amygdaloidal, and it has been described as such. 
Microscopic description. — Microscopic examination of sections cut 
'rom the finer-grained specimens collected at this place showed their 
mmistakable tuffaceous character, but they were too decomposed to 
be studied in detail. The coarser tuff, however, was seen to consist of 
|!*ounded, pumiceous areas set in a groundmass composed almost 
mtirely of minute lapilli, with their characteristic jagged and concave 
mtlines. The spherical masses also are composed of minute glassy 
j 'ragmen ts. In addition to these glassy components, the rock contains 
i few feldspar crystals, some fragments of mica, and areas of 
ievitrified glass. 
The rock is but slightly stratified, if at all. The fragments are all 
mall and very angular, and minute glassy particles are abundant. 
These facts indicate an air-laid breccia. 
TUFF ON THE ASHLAND-SHERIDAN ROAD. 
The bed of volcanic tuff near the Ashland-Sheridan line appears 
s a dark-gray, mottled rock of light and dark areas, the nature of 
vhich is not evident in the hand specimen. Under the hammer it 
>reaks into irregular fragments, and is traversed by a set of cleav- 
ge planes. Where it is much weathered the rock is not distinguish- 
ble on the surface from the volcanic sandstone with which it is 
ssociated. 
