20 
VOLCANIC PRODUCTS 
BOOK I 
or straight cracks between which the vitreous or felsitic substance, during its 
contraction in cooling, assumed a finely globular form. 
Another structure, termed Spliervlitic (Fig. 5), shows the development 
of globules or spherules which may range from grains of microscopic minute- 
ness up to balls two inches or more in diameter. These not infrequently 
present a well-formed internal fibrous radiation, which gives a black cross 
between crossed ISlicol prisms. Spherulites are more especially developed 
along the margins of intrusive rocks, and may be found in dykes, sills and 
bosses (see Figs. 375 and 377). Where the injected mass is not thick it 
may be spherulitic to the very centre, as can be seen among the felsitic 
and granophyric dykes of Skye. 
Some felsitic lavas possess a peculiar nodular sti'ucture, which was 
developed during the process of consolidation. So marked does this arrange- 
ment sometimes become that the rocks which display it have actually been 
mistaken for conglomerates. It is well exhibited among the Lower Silurian 
lavas of Snowdon, the tapper Silurian lavas of Dingle, and the Lower Old 
Eed Sandstone lavas near Killarney. 
A marked structure among some intrusive rocks, especially of an acid 
composition, is that called Micropeg'inatitic or Granopliyric. It consists in a 
minute intergrowth of two component minerals, especially (j^uartz and felspar. 
Fig. 6. — Mieropegiuatitic or Granopliyric 
structure in Grauopliyre, Mull (magnified). 
Fig. 7. — Opliitic structure in Dolerite, 
Gortacloglian, Co. Derry (niagnitted). 
and is more especially characteristic of certain granitic or granitoid rocks 
which have consolidated at some distance from the surface and occur as 
bosses, sills and dykes. It is also met with, however, in some basic sills. 
Examples of all these and other structures will occur in the course of the 
following description of British volcanic rocks. 
(2) The second type of devitrification, conspicuous in rocks of more 
basic composition, is marked by a more complete development of crystal- 
lization. Among basic, as among acid rocks, there are proofs of the consoli- 
dation of definite minerals at more than one period. Where the molten 
material has suddenly cooled into a black glass, porphyritic felspars or 
other minerals are often to be seen which were already floating in the 
