28 o 
VOLCANOES OF THE LOWER OLD RED SANDSTONE 
BOOK V 
Fioni these analyses it may he interred that the average amount of 
silica in the more typical varieties is between 70 and 75 per cent. The 
last specimen in the table, with its abnormally high percentage of acid, 
must he regarded as an exceptional variety, where there has either been an 
excessive removal of some of the bases, or where silica has been added by 
infiltration. 
The microscopic examination of these rocks lias not added much to the 
information derivable from a study ot them in the field. In their most 
close-grained varieties, as above remarked, they are hardly to be dis- 
tinguished from felsites. But they generally show traces of the minute 
detrital particles ot felsite of which they are essentially composed. The 
brecciated varieties exhibit finely-streaked flow-structure in some of the 
fragments. Pieces of andesite, grains of quartz, and other extraneous 
ingredients appear in these rocks towards the southern limits of the 
volcanic area of the Pentland Hills, where the acid tuffs are associated 
with and pass laterally and vertically into ordinary non-volcanic sedi- 
mentary strata. Further details as to the part which these tuffs play in 
the volcanic history of the regions wherein they occur will be given in 
later pages. 
