Oct., 1889. 
PETROLOGY OF LOCAL PEBBLES. 
237 
continuous through two neighbouring grains without any 
trace of them being visible in the crystals of the ground 
mass. The arrangement of the quartz and felspar in this 
ground mass shows in places an approximation to pegmatite 
arrangement, and in some cases the boundaries of the quartz 
seem indented, as it were, by the quartz of the ground, as if 
the crystallisation of the latter had been regulated by the 
previously existing quartz. 
The felsites which occur among our pebbles are of two 
or three principal types. There are reddish-brown quartz 
felsites, grey rhyolitic specimens, and others of the grey, 
and esitic type, which are so more plentiful in Wales. 
(14) At Sutton, King’s Heath, and also at Catspool, near 
Alvecliurch, there occur pebbles of red quartz felsite, with, 
in most cases, abundant porphyritic quartz and felspar 
crystals. Many of the latter are still fairly fresh, and show 
bright cleavage faces on a fractured surface. 
The micro-structure is very similar in all cases, the 
ground mass being a micro-granite of rather fine grain. The 
quartz has the deep bays and tongues of the ground mass 
running into the grains which are so common in the felsites. 
The felspar is partly at any rate triclinic, and in some 
sections not much is left but the mere outline or ghost of the 
crystal. 
(15) A specimen collected at a little digging by the 
Keeper’s Pool, at Sutton, on account of the large number 
of felspar crystals, the absence of visible quartz, and the 
presence of a good many flakes of black mica, presents a very 
different aspect from that of the felsites already mentioned, 
and we find that under the microscope the same differences 
are apparent. The felspars are to a very great extent 
unaltered, and both orthoclase and a plagioclase are present. 
The mica occurs in good quantity, and lias sometimes 
undergone a change into a pale green substance, which also 
has replaced hornblende, the shape of the original mineral 
being retained. 
The ground mass is very finely micro-crystalline. 
In the next group of felsites we have still different 
structure. 
(16) In a pebble, from Catspool, there is very decided 
flow structure in the ground mass, not straightforward 
streams, but presenting the crumpled up, damascened 
appearance, which denotes the attempt at mixture of two 
slightly different viscid magmas, becoming stiffer as they cool 
and are pressed on from behind by the flow of the mass. 
The quartz grains are small and numerous, and the felspar 
is much altered. 
