MUSEUM OF COMPARATIVE ZOOLOGY. 119 
bombs. Above the tufaceous deposit is a sheet of very amygdaloidal 
trap, overlain by a dark pinkish gray sandstone, carrying two thin sub- 
ordinate layers of trappy material a few feet over the contact. In the 
hand specimen and under the microscope, this sandstone appears identi- 
cal with the fine matrix of a trap conglomerate noted by Percival as 
occurring half a mile to the north, and presumably forming the strati- 
graphical equivalent of the tufaceous deposit at this point. The se- 
quence of outcrops here disclosed is one of the most valuable that it has 
been our fortune to discover, and has attracted much local attention 
since it was found in the spring of 1887. It will well repay attentive 
' examination. The following account refers in greater part to its micro- 
scopic structures. 
Under the microscope the material of the bluff enclosing the volcanic 
bombs is found to be made up of small fragments of trap, generally very 
fine-grained and much altered. Small greenish brown areas dotted 
thickly with ferrite are non-polarizing as a whole ; these appear to be 
volcanic glass. A few porphyritic ledges of plagioclase occur in them. 
Most of the eruptive grains have been altered to chlorite and quartz, 
and are intimately mixed with granular calcite. The microscope fails 
to discover any grains of water-worn quartz or other clastic material, 
although it is probable that more or less normally deposited sediment 
occurs thinly scattered through tlie mass. No stratified arrangement 
of the trap grains is noticeable in the microscopic sections, except an 
orientation of chlorite plates parallel to the stratification of the sand- 
stone on the back of the ridge, and to a rude lamination brought to sight 
in the face of the tufaceous bed by weathering. Following Geikie, this 
bed would be called a tufa, consisting of a shower of lapilli. It appears 
to have been deposited rather rapidly in a body of water, and probably 
at no great distance from a point of eruption, as it soon disappears to 
the north and west. It is traceable.a mile and a half to the southeast, 
in localities 6 and 7. 
The volcanic bombs occurring with the lapilli give the face of the 
bluff a curious mottled appearance. They show no definite arrangement, 
but are more numerous near the bottom of the bed, where one of them 
seems to have imbedded itself in the underlying sandstone ; they are 
remarkable for their non-vesicular character and their compact uniform 
texture from the centre to the surface. The microscope detects no 
variation in texture in any part except that due to a partial alteration 
of the surface. It shows them to be extremely close-grained, with por- 
phyritic crystals of augite set in a ground mass of minute plagioclase 
