88 PROCEEDINGS COTTESWOLD CLUB 1903 
A still more remarkable instance of this re-building of 
quartz crystals I discovered in residue left after dissolving 
portions of Carboniferous Limestone in acid. I may say 
that in most limestones there is a certain amount of 
detrital material derived from the denudation of the land 
and carried out to sea in the form of mineral grains. 
Now, in the case of the Carboniferous Limestone residue 
there were original grains of quartz in which, by reason of 
the deposition of secondary silica, the crystalline form 
had been completely restored. The original quartz grain 
appears in the centre like a nucleus and the restored faces 
of the crystal can be completely traced. This process of — 
the restoration of worn crystals is very remarkable, and 
there is a similar tendency to return to the crystalline 
condition in the case of amorphous silica. This amorph- 
ous silica is frequently to be found in insoluble residue 
from limestones, resulting from the decomposition of 
felspar and other minerals, and the tendency is to return 
to the original and stable crystalline condition. 
To return for a moment to the Millstone Grit, there is 
an instructive lesson to be deduced from the two types of 
the rock, the Bristol and Forest of Dean. It is that we 
have rocks of the same age differing lithologically, and 
thus showing the danger of determining rocks from their 
lithological character only. 
Coming now to the calcareous deposits of former days, 
among the oldest series of limestones of which we have 
knowledge in England are those which occur in the 
Ordovician system, or Upper Cambrian rocks of Sedgwick. 
Needless to say, the Ordovician rocks are near the bottom 
of the geological ladder, so far as we know it, and are 
therefore extremely old. Some are also greatly altered 
by time and natural process of mineralisation. Among 
the limestones of this age are those of the Bala group, 
which, as their name implies, are developed in the neigh- 
bourhood of Bala, North Wales. The rocks are chiefly 
a 
