88 STRUCTURAL AND FIELD GEOLOGY 
having as common associates six-sided prisms of nepheline, and not 
infrequently prisms of some pale green pyroxene and small crystals of 
magnetite. But dark-coloured minerals are not as a rule so common 
as in trachyte. The rock is prone to become decomposed, the decom- 
position-products, in the form of various zeolites, appearing in cracks 
and cavities. Quartz-porphyries, which, to the unassisted eye, may seem 
to consist exclusively of crystalline ingredients, have a granitoid aspect. 
On fresh faces the observer will readily distinguish the two dominant 
minerals, orthoclase and quartz, while on weathered faces the presence 
of a groundmass is often revealed by earthy or clay-like matter entangled 
between the quartz and the weathered felspar. 
(dz) Coarse-grained Holocrystalline Felspathic Rocks offer on the 
whole fewer difficulties to the beginner. If he has already learned to 
recognise the common rock-forming minerals, he should be able to 
distinguish the several essential constituents of such rocks as coarse- 
grained granite, syenite, diorite, gabbro, and dolerite. The finer grained 
holocrystalline varieties will sometimes be diagnosed: with difficulty. 
In such cases he will often be aided by the appearances presented by the 
weathered crusts. The felspars, he will remember, tend to be decomposed 
into an earthy or clay-like substance, which will be lightly or more 
darkly tinted according to the proportion of decomposing ferromagnesian 
constituents with which they are associated. 
(ec) Fragmental Igneous Rocks vary greatly as regards texture, some 
being exceedingly fine-grained, while others are composed of an aggregate 
of larger and smaller blocks. The finer grained varieties are usually of 
an earthy or clay-like aspect and readily scratched ; many, indeed, are so 
slightly compacted that they may be disintegrated between the fingers. 
Such rocks may show scattered through them flakes of mica, and broken 
crystals of various volcanic minerals. They are usually well-bedded, 
having been arranged and spread out in layers by aqueous action. For 
the same reason they often dovetail with or pass into ordinary aqueous 
rocks, such as sandstones and shales. Many tuffs, again, consist largely 
of finely comminuted débris of igneous rocks, either of one or of different 
kinds ; such tuffs commonly contain cinders and lapilli of lavas. Rocks 
of this class are most usually interbedded with lava-form igneous rocks. 
The coarser agglomerates and volcanic breccias may also occupy a 
bedded position, but they very frequently occur in the pipes of old 
volcanoes or in masses immediately surrounding these. 
(f) Crystalline Schists.—Their determination is, as a rule, not hard. 
Where the several ingredients are conspicuous, and the rock tolerably 
fresh, its diagnosis should not be more difficult than that of a coarse- 
grained holocrystalline igneous rock. Not infrequently, however, the 
component crystals of a schist are very small individuals, and so closely 
intermingled, that they can hardly be distinguished even with the help 
of one’s pocket-lens. In such cases, should the rock be a pale whitish- 
green colour, have a marked soapy feel, and be easily scratched, it will 
probably be ¢alc-schist. Chlorite-schist is also easily scratched, but is 
dark green and not quite so ynctuous to the touch. //ydro-mica-schist is 

