92 RASTALL : THE INGLETONIAN SERIES OF W. YORKSHIRE. 
Professor Bonney.* I have therefore examined a large number 
of shces obtained from numerous locaUties, and the results 
appear to be of considerable interest, since the series as a whole 
presents certain peculiarities which may be expected to throw 
some light on the origin and possibly also on the age of the rocks. 
As has been already mentioned, the Ingletonian rocks 
comprise a great thickness of massive grits and well-cleaved 
slates. These, however, are somewhat extreme types and 
many of the beds are intermediate in character, and may be 
regarded either as very fine grits and flags or rather coarse 
slates. As would naturally be expected, the effects of pressure 
are most apparent in the finer sediments, while the coarse grits 
are merely affected by systems of joint planes. Another im- 
portant feature is that in many cases the beds of finer texture 
have been crushed between the hard masses of grits, and are 
often more or less contorted. Since the grits of medium texture 
are perhaps the most abundant and most typical, it will be well 
to begin our study of the petrography of the series with them. 
The Grits. 
An average specimen of such a grit is of even texture, 
generally of a greenish or bluish colour, and usually showing 
a certain amount of pink felspar and flakes of white mica. 
In fact, the presence of a considerable amount of detrital mica 
is one of the characteristics of this series. 
When examined under the microscope these grits are seen 
to consist for the most part of angular grains of quartz and 
felspar, and flakes of muscovite ; there is also a fair proportion 
of compound grains, consisting for the most part of fragments 
of quartzite and other metamorphic rocks. The proportions 
in w^hich the different constituents occur are very variable, 
but in all cases felspar and mica are present to a notable extent. 
The quartz grains vary much in size, and most of them are 
distinctly angular, often unusually so ; some of the larger grains 
are to a certain extent rounded. Much of the quartz shows 
* British Association Report, 1886. 
