CHAPTER XL VI 
TYPES OF STRUCTURE IN THE ACID ROCKS BOSSES 
Returning now to the consideration of the acid rocks as these manifest 
themselves in the volcanic areas of Britain, I would remark that three 
distinct types of structure may be noted among them, viz. (1) bosses, (2) 
sills or intrusive sheets, (3) veins and dykes. These types, as above re- 
marked, belong entirely to the underground operations of voleanism, for 
though the rhyolitic fragments in the tuffs and agglomerates of the plateaux 
prove that acid lavas existed near the surface, no undoubted case of super- 
ficial lava belonging to the acid series has yet been observed. 1 
Die bosses of acid material in the British Tertiary volcanic series 
are irregular protrusions, varying in size from knobs only a few 
square yards in area up to huge masses many square miles in extent, and 
comprising groups of lofty hills. As a rule, their outlines are markedly 
irregular. Beneath the surface they plunge down almost vertically through 
the rocks which they traverse, but in not a few instances their boundaries 
are inclined to the horizon, so that the contiguous rocks seem to rest against 
them, and sometimes lie in outliers on their sides and summits. From the 
margins of these bosses apophyses are given off into the surrounding rocks, 
sometimes only rarely and at wide intervals, in other places in prodigious 
numbers. Sometimes the acid material has been injected in thousands of 
veins and minute threads, which completely enclose fragments of the sur- 
rounding rock. 
The rock of which the bosses consist is generally granophyrie in texture, 
passing on the one hand, particularly in the central parts, into granite, and 
on the other, and especially towards the margin, into various more compact 
felsitic varieties, and sometimes exhibiting along the outer edge more or less 
developed spherulitic and flow-structures. 
Decided contact metamorphism is traceable round the bosses, but is by 
no means uniform even in the same rock, some parts being highly altered, 
while others, exposed apparently to the same influences, have undergone 
1 The rhyolites ot Tardree in Antrim have recently been claimed by Professor Cole as true 
lavas grouped round an eruptive vent. For reasons to be given in the next chapter I regard 
them as intrusive masses, though they may not improbably have been connected with streams of 
lava now entirely removed. 
