414 SIMPSON : STRATA AND DEPOSITION OF THE MILLSTONE GRITS. 
mode of deposition, in the light of which we may grasp the pheno- 
mena better. Let us first just pass in review again the class of sand- 
stones comprising the series. They range from the extremely coarse 
and conglomeratic gritstone to the finest grained and most compact 
sandstone. The rougher examples are generally thickly and most 
irregularly bedded, whilst the finer ones are minutely and most 
evenly laminated. Most of them, however, coarse and fine alike, 
show traces of current-bedding. Where they are unweathered they 
are generally blue or grey, and contain iron as ferrous carbonate. 
Where they are exposed they are of various shades of ruddy- brown 
or yellow, the change being produced by the action of the air and 
water on the ferrous salt ; the carbonate becomes changed into one 
of the ferric hydrates, and the exact colour, in all probability, 
depends on the proportion of combined water. In all the sandstones, 
however, the constituents may generally be said to be alike, viz., quartz, 
felspar, and mica. The mica is displayed along the planes of lamina- 
tion of the finer sandstones very plentifully, and is naturally more 
rarely seen in the coarse grits. 
Dr. H. C. Sorby, who has made careful examination of these 
rocks, is of opinion that the felspar has, in the first place,been present 
in grains, crystals, or pebbles, as a felspar sand ; has been decom- 
posed after deposition, re-deposited as a paste, and squeezed by pres- 
sure into the interstices between and round the quartz grains. 
Dr. Sorby describes, in another paper read before the Micro- 
scopical Society, instances of the repairing of sand grains by a process 
of secondary crystallization. Whilst doubtless the angularity of many 
of the quartz grains in the grits is original (as sand grains being- 
smaller than pebbles would not be subject to so much attrition and 
thus not so rapidly rounded) ; I have still been inclined to think, 
although without actual knowledge, that we may owe the 
extreme angularity of some of the grains to a process of secondary 
crystallization ; decomposing felspar would furnish the necessary 
silica, which might have been partially used thus in the repairing of 
the crystals. 
I mentioned before that the Grit series is interesting as illus- 
trating a physical change in the relative conditions and distribution of 
