RASTALL : THE INGLETONIAN SERIES OF W. YORKSHIRE. 93 
distinct strain-shadows. The felspar includes several varieties, 
orthoclase, plagioclase, and less commonly microcline and 
perthite. Perhaps the most abundant is plagiolcase, and this 
usually shows straight extinction or nearly so, so that the domi- 
nant variety appears to be of an acid nature. Basic plagioclase 
is apparently absent. In all slices there are to be seen abundant 
flakes of mica, of all sizes ; the muscovite is fresh, as is usual 
in sediments, and much of the chlorite which is so common in 
these rocks appears to be derived from biotite. Occasionally 
almost unaltered crystals of this mineral can be seen. These 
grits often contain many crystals of heavy minerals, such as 
iron ores, sphene, epidote, zircon : in fact, all the minerals which 
commonly occur as accessories in acid igneous rocks. 
All these constituents are embedded in a cement which 
varies somewhat in composition, but is usually to be described 
as a mosaic of minute grains of quartz, enclosing tiny flakes of 
mica and often a certain amount of calcite. In a few specimens 
the proportion of calcite is so high that the rock almost deserves 
to be called a calcareous grit. All these rocks contain more 
or less green chlorite in various forms, sometimes finely dis- 
seminated through the cement, and at other times occurring 
in more or less idiomorphic crystals, which probably result 
from the decomposition of ferromagnesian minerals. The 
prevailing and characteristic green colour of these rocks is due 
to this mineral. The chlorite is undoubtedly of secondary 
origin, and much of it was introduced subsequently to the de- 
position of the rock, since the same mineral occurs very largely 
in the veins of both quartz and calcite which traverse the rocks 
in many localities (see Plate VIII.). 
Different specimens of the finer grits show a good deal of 
variation in detail, but all may be referred to one general type, 
viz., a felspathic grit or arkose, with a notable amount of detrital 
mica, and much stained by chlorite. The angularity of the 
fragments is very noticeable, and suggests rapid deposition 
under conditions which precluded much rolling and attrition 
of the fragments. It is also worth mentioning that the felspar 
is very fresh, as much so as in the Torridonian arkoses. 
