VOL. XIV.(2) |THE PRESIDENT’S ADDRESS 87 
classed as a quartzite, which may be defined as a hard 
sandstone, usually very firm and more or less crystallised. 
The Millstone’ Grit in the Forest of Dean is very 
different except in actual chemical composition. Instead 
of being a hard rock it is a soft sandstone, and there is so 
little cohesion between the grains that it is difficult to get 
a section owing to the ready way in which the rock falls 
to pieces. Seen under a microscope the quartz grains are 
found to be well rounded by water action. On the other 
hand, examination of the separated quartz grains from the 
Bristol Millstone Grit shows them to be somewhat 
angular, but what is most interesting is that around the 
edges of the grains of quartz secondary crystalline silica 
appears, and it is this feature which accounts for the com- 
pactness of the Bristol Millstone Grit. In other words, 
the spaces between the quartz grains have been closed up 
by the deposition of secondary silica, and the mineral 
grains cemented together, as it were. But this is not all; 
the most interesting part of the story is the way in which 
this secondary silica has become deposited. Examination 
of the original quartz grains, both in the Bristol and 
Forest of Dean Millstone Grit, shows that many of them 
still retain rough outlines of crystals, the sharp angles of 
which have been worn off. That is to say, the evidence 
appears to show that the quartz grains were derived from 
the denudation of crystalline rocks, such as granite. Now, 
the sharp angles of these crystals, as I have said, were 
more or less worn off during the process of deposition, 
more especially in the case of the Forest of Dean Mill- 
stone Grit. But, apparently by the law of chemical 
affinity, the old, worn quartz grains have, if I may so 
express it, tried to replace their damaged outlines and to 
reappear as defined crystals. Examination of the grains 
shows that the secondary silica is deposited on the edges 
of the damaged crystals, and in some cases the original 
crystalline form is restored. 
