26 
compared with the same formation exposed round Bristol. 
Considering the close proximity of the outcrop of the northern 
end of the Bristol coal-field, I was certainly not prepared to 
find so great an alteration in the grit. I may state, however, that 
lithological characters cannot always be absolutely relied upon 
for determining the age of rocks, and great mistakes have been 
made on that account. Throughout geological time we get a 
recurrence of similar physical conditions, and the lithology 
of the rocks simply bears evidence as to what those conditions 
were, and even here we have to take into consideration 
the changes which have been brought about by the chemical 
affinities of the substances composing rocks, and by the per- 
colation of water through them. There are however certain 
special lithological peculiarities exhibited by some rocks which 
may, at any rate, be taken into consideration over a small area. 
Thus, in the present case, the Millstone grit of the Bristol 
coal-field is composed of grains of quartz which appear to 
cement themselves together (Fig. II.) In the specimen figured, 
there is a little ferruginous substance to be seen between the 
grains, but in specimens which I have in my collection, the 
appearance presented is that of a mass of quartzite, and one 
has great difficulty in distinguishing the grains, so closely are 
they cemented by the cohesion of the particles of one grain 
with another. Again, the grains are sub-angular, a character 
which is not confined to the Bristol district. Dr. Sorsy, F.R.S., 
in his Presidental address to the Geological Society in 1880,* 
referring to the Millstone grit around Sheffield, said: ‘The 
grains of quartz are, on the whole, extremely angular, and as a 
general rule show little trace of wearing.” 
When, therefore, I first saw the bed, under consideration, I 
naturally hesitated before coming to a conclusion as to its 
stratagraphical position. The matter was, however, decided 
on a second visit to the Drybrook section, when I found a 
Lepidodendron in a quarry opened on a bed of a yellow colour, 
about half a mile from the section at Morse, but in other respects 
* Proceedings Geological Society, 1880, page 64. 
