240 
PETROLOGY OF LOCAL PEBBLES. 
Ocx., 1889. 
running nearly parallel to each other through the rock give 
a well-marked flow structure to the specimen. 
On a surface tangential to the flow they appear as broader 
plates. 
Examined by the microscope in thin section they are of a 
rusty-yellow colour, and are composed of crystalline masses, 
the formation of which has apparently gone on from the 
edges. The substance has low double refraction and no 
visible dicliroism, and seems to be what is generally called, 
for want of more knowledge as to its real nature, viridite. 
It has the appearance of filling cavities in the original rock. 
The larger felspars are mostly striated, and are in fairly 
good condition. 
The ground mass is mainly composed of interlacing 
felspar crystals of minute size, but among them are the 
appearances of a residual glass, now devitrified and showing a 
pale indefinite mosaic between crossed nicols. 
(27) A thoroughly good gneiss (23) occurs among the 
Sutton pebbles. It is composed of quartz, felspar a good 
deal decomposed, and a pale mica. 
In one part of the section lines of inclusions run almost 
all across the slide at nearly right angles to the foliation, 
showing their origin to have been subsequent to the formation 
of the gneiss. 
(28) Of somewhat similar character is a pebble from 
Sidnal farm (49). 
It is coarser in grain, but consists essentially of quartz 
felspar and white mica. The foliation is not by any means 
marked. 
It contains a number of collections of black, quite opaque 
grains, which show no crystal form, and give no indication of 
their nature. 
(29) A dolerite occurs at Catspool, and also in one or two 
other places in the district near (46). 
It contains olivine, and differs from the Eowley Eag by 
being ophitic in structure. 
At Sidnal farm the little gravel pit furnished, in addition 
to the rocks already mentioned, a very coarse conglomerate 
of quartz and felspar fragments set in a finer matrix. The 
felspar appears to be ortlioclase, and shows wonderfully 
brilliant cleavage planes. 
At Catspool a pebble of red granite, and one of a red 
syenite were found, and one of slate traversed by a quartz 
vein. 
At the Ox Leasowes pit a mass of chert, with casts of 
encrinites, was found very similar to those which are met with 
at Sutton. 
