172 
MELLO: MTCRO^rOPKAL STRUCTURE OF ROCKS. 
quartzites, clays, shales, and slates and schists. The prevalent 
minerals in these yiiW be found to be quartz, mica, and various 
species of felspar; the felspars, as well as other minerals occasion- 
ally seen, are frequently decomposed and pseudomorphosed. In 
sandstone and grit rocks a microscopical examination of the condi- 
tions of the structural grains will afford some clue as to their antiquity; 
some sands will present gi-ains only partially worn, whilst others will 
be built up of more or less angular grains: by observations of this 
sort Dr. Sorby has pointed out that we may " learn whether sand is 
of recent and comparatively local origin, or of very ancient, and 
transported far from its original source by drifting along the bottom." 
The presence again of fractured gTains will sometimes " indicate 
some unusually violent local action."* 
Slates need to be very closely examined in order to ascertain 
their character, whether those we are investigating are a mere 
aqueous sediment, formed by the disintegi'ation of older rocks, or 
whether they are a volcanic ash. The microscope applied to the 
coarser varieties will enable us to decide the question; also, whether 
as regards the former class, the slate was derived from a coarse- 
grained granite or felsite, or from a fine-grained micaceous rock: the 
coarser rock would. Dr. Sorby has shewn, yield a kaolinitic clay-slate, 
wliilst the finer one would originate a micaceous clay-slate, since the 
mica in such fine-gTained felsites, etc., would not be separated from the 
kaolin, as it would be where occurring in larger flakes or leaflets, 
v.hich would l)e left behind with the quartz grains as sediments were 
formed to produce micaceous sandstones and grits. 
In the case of an ash, the coarser portions of a slate will show 
that it do3s not consist merely of the broken fi-agments of some 
older rock, but there will often be direct evidence of an igneous 
origin, such as the presence of pumiceous material, whilst numerous 
gas cavities will be found both in the felspathic crystals and in por- 
tions of the base. In some of the slates of the North of England 
pumice occurs in fairly large fragiuents, having its cavities filled with 
calcite. 
* Q. J. G. S , vol. xxxiv., part 2. " On the Structure and Origin of Noa* 
calcaieous Stratified Hocks," by Dr. Sorby, F.R.S , etc. 
