366 
THE TERTIARY VOLCANOES 
BOOK VIII 
i. PETKOGKAPHY OF THE ACID KOCKS 
The classification of the rocks which best harmonizes the field-evidence 
and the detailed study of their mineralogical composition, is one that arranges 
these volcanic protrusions into two series. In the one, the orthoclase is 
sanidine, and the rocks range from the most vitreous pitchstone through 
perlitie and spherulitic varieties to rhyolite (“ quartz-trachyte ”). In the 
other series, which embraces by far the largest proportion of the whole, the 
orthoclase is always turbid, and in this respect as well as in many others the 
rocks remind us rather of ancient eruptive masses than of those which have 
appeared in Tertiary time. They range from fiinty felsitic varieties, which 
are obviously devitrified glasses, through different textures of quartz-porphyry 
into granophyre, and finally into granite. As I have been unable to recog- 
nize any essential difference of structure and composition between these acid 
Tertiary rocks and those of far earlier geological time, I give them the names 
which no petrographer would hesitate to apply to them if they were of 
Palaeozoic age. It has long appeared to me that these rocks furnish con- 
clusive evidence of the misleading artificiality of any petrographical nomen- 
clature in which relative antiquity is made an essential element of discrimi- 
nation. 
Granite . — That true granites form part of the Tertiary volcanic series 
of the British Isles has now been completely established. They occur as 
bosses and sills which have been intruded into the gabbros and all older 
rocks. They are thus proved not only to belong to the Tertiary period, 
but to one of the latest phases of its volcanic history. But besides these 
granites, the relative age of which can be definitely fixed, there occur others 
which, standing alone and at some distance from the basaltic plateaux, can 
only -be inferentially classed in the Tertiary series. To this group belong 
the granite masses of the Isle of Arran and the Mourne Mountains in north- 
eastern Ireland. 
Taking first the unquestionably Tertiary granites which occur as bosses 
and intrusive sheets, we have to note that the more coarsely crystalline 
granophyres are hardly to be distinguished externally from granite. As the 
dark ferro-magnesian constituent of these rocks was generally believed to be 
hornblende, they were called by the older petrographers “ syenite ” ; that is, 
granite with hornblende instead of mica. The peculiar micropegmatitic 
groundmass, which constitutes the distinguishing feature of the granophyres, 
may occasionally be observed so reduced in amount as only to appear here 
and there between the other minerals, which are grouped in a granitic 
structure. From this condition, one step further carries us into a true 
granite, from which all trace of the granophyric character has disappeared. 
Such gradations may be traced even within short distances in the same 
boss of rock. Thus, in the hornblende -biotite- granite boss of Beinn-an- 
Dubhaich, Skye, a thoroughly granitic arrangement of the component 
minerals is observable in the centre, while a specimen taken from near the 
edge on the shore of Camas Malag shows the development of a grano- 
