SOME NORTH-COUNTRY QUARTZITES. 
ALFRED HARKER, M.A., F.G.S. 
UNDER the name quartzite are included rocks which, while having 
the same general characters, have originated in more than one way. 
Confining ourselves to the limited and local occurrences of quartzites 
in the North of England, we may say that, in every case, the rock 
has once been a quartz-sandstone or grit, and has been altered to its 
present condition. The original fragmental nature has been partially 
obscured by the constituent grains becoming less distinct to the eye, 
and the whole has become compacted into a more or less homo- 
geneous, stubborn rock. There are two different processes by which 
this result may be brought about, and although quartzites of the two 
classes may appear almost identical in a superficial view, the 
distinction becomes in most cases very apparent when thin slices 
of the rocks are examined under the microscope. The two 
processes are cementation and metamorphism. 
It has been more clearly recognised of late years than formerly, 
that many quartzites have been produced without the operation of 
anything that is implied in the ordinary usage of the term meta- 
morphism ; that is, without either high temperature or great i Be 
The interstices between the original grains of such a rock have bee 
simply filled by new quartz deposited from solution in water. That 
the percolating water which carried this silica in solution was not 
necessarily at a high temperature is proved by the fact that the 
process of cementation is actually going on at the present day in 
some localities, where exposed surfaces of sandstone or grit are seen 
to be superficially shinier into glassy-looking or enamel-like 
quartzite. The microscope shows that, in some cases, the new 
quartz is deposited entirely inept of the old grains ; very 
often, however, the old grains themselves grow by the deposition on 
€ach of new material, which attempts to reconstruct the crystal ot 
which the grain was a fragment. The grains grow until they inter- 
fere with one another, and the interstices are entirely filled in. 
Optical examination of a thin section of a quartzite thus formed, 
Shows that the fringe of new quartz surrounding any original grain is 
in crystalline continuity with it; but the more limpid appearance of 
the new deposit usually enables the outlines of the old grain to be 
detected. Some of our Carboniferous sandstones have been con- 
verted into quartzites of this type. An excellent example occurs in 
cca basement beds of that formation on Roman Fell, near —— 
arch 1892, | 
