36 STRUCTURAL AND FIELD GEOLOGY 
mineral bodies interfering with each other’s growth and thus forming a 
close aggregate, diffused through which glassy matter might occur either 
sparingly or abundantly. This is the so-called groundmass. When 
phenocrysts are conspicuously present in it we have what is known as 
porphyritic structure (see Plates X.; XII. 1). It will be understood, 
therefore, that such porphyritic rocks give evidence of two stages of con- 
solidation—the phenocrysts belonging to the earlier or zz¢vatelluric, and 
the groundmass to the final or volcanic stage. 
The groundmass of hemicrystalline rocks is as a rule mostly made 
up of crystalline ingredients. In some of these rocks, however, it 
consists chiefly of glass, while in many others crystalline constituents and 
glass are approximately equal in amount. The non-differentiated matter 
or dase of the groundmass not infrequently assumes a szcrofelsztic 
or cryptocrystalline character. To the unassisted eye this substance 
seems to be quite compact and homogeneous; but under the micro- 
scope microfelsitic matter appears as an indefinite aggregate, or nearly 
structureless mass. 
As might have been expected, the non-differentiated matter in the 
groundmass frequently shows the structures which have already been 
described as characteristic of volcanic glass. It is often more or less 
devitrified and stony-like, owing to the abundant development of 
crystallites and microlites, while spherulitic and perlitic structures are of 
common occurrence. Fluxion structure also is often seen, not only in the 
base but throughout the whole groundmass. Lastly, vesicular structure, 
as already indicated, is just as characteristic of hemicrystalline as of 
vitreous rocks. 
Holocrystalline Rocks. Zheir General Character.—These rocks con- 
tain no non-differentiated matter—they have no base, and as a rule no 
proper groundmass (see Plate XII. 2). Not infrequently, however, they 
show conspicuous phenocrysts disseminated through the relatively fine- 
grained crystalline aggregate which constitutes the mass of the rock. 
‘Such rocks would therefore seem to have experienced two stages of solidi- 
fication—the phenocrysts, as usual, having crystallised out first. Both 
stages of solidification, however, were intratelluric — holocrystalline 
rocks being usually of more or less deep-seated origin. They differ 
greatly in texture, some being very finely crystalline, while others are 
exceedingly coarse-grained, and between these extremes all intermediate 
textures occur. 
Mineral Ingredients of Igneous Rocks.—Many different minerals enter 
into the composition of igneous rocks, anhydrous silicates being by fai 
the most important. Save in the case of well-developed phenocrysts 
and the smaller accessory ingredients, these minerals are not as a rule 
completely bounded by crystal faces. When they are thus bounded they 
are said to be exzhedral or tdiomorphic. Should only some of the crystal 
faces appear (and this is very often the case with those minerals which 
were the first to crystallise out after the phenocrysts and smaller 
accessories had appeared), then the structure is termed szdhedral or 
hypidiomorphic. Most commonly, however, the minerals, owing to 

