CHAPTER XLV 
THE ACID BOCKS 
Their Petrography — Their Stratigraphical Position and its Analogies in Central France 
We now come to the examination of another distinct phase of volcanic 
action during Tertiary time in Britain. The igneous rocks that have been 
under consideration in the foregoing chapters, whether poured out at the 
surface or injected below ground, have been chiefly of basic, partly indeed, 
like the peridotites, of ultra-basic character. Some, however, have shown an 
andesitic or intermediate composition. Reference has also been made to the 
probable eruption of acid rhyolites in the long interval between the outflow 
of the lower and the upper basalts in Antrim. But we now encounter a 
great series, decidedly acid in composition, in the more largely crystalline 
members of which the excess of silica is visible to the eye in the form of 
free quartz. While there is a strong contrast in chemical composition 
between this series and the rocks hitherto under discussion, there are also 
marked differences in structure and mode of occurrence. Like the gabbros, 
all the masses of acid rock now visible appear to be intrusive. They have 
been injected beneath the surface, and therefore record for us subterranean 
rather than superficial manifestations of volcanic action. 
The existence of rocks of this class in the midst of the basic masses has 
long been recognized. They were noticed by Jameson, who described the 
hills between Loch Sligachan and Broadford as composed of “ a compound of 
felspar and quartz, or what may be called a granitel, with occasional veins 
of pitchstone .” 1 Macculloch gave a fuller account of the same region, and 
classed the rocks as chiefly “ syenite ” and “ porphyry .” 2 In Antrim, also, 
even in the midst of the basalt-tableland, masses of “ pitchstone-porphyry,” 
“ pearlstone-porphyry,” “ clay-porphyry,” and “ greystone ” were observed and 
described . 3 In more recent years Professor Zirkel has given a brief account 
of the so-called “ syenite and porphyry ” of Mull and Skye , 4 and the late 
1 Mine.ralogicf.tl Travels, ii. 90. 
3 Western Isles, see the descriptions of Skye, Mull and Rum. 
3 Berger, Trans. Geol. Soe. iii. (1816), p. 190 ; Portlock, Journ. Geol. Soc. Ireland, vol. i. 
(1834), p. 9. 
4 Zcitsch. Deutsch. Geol. Gesellsch. xxiii. (1871), pp. 54, 77, 84, 88. 
