
ROCKS 87 
divided by prismatic joints, and thus rendered columnar in structure. 
Basalt weathers with a thick rusty or yellowish-brown crust, which often 
exfoliates in concentric shells. Not infrequently, owing to the alteration 
of its ferromagnesian constituents into serpentine and chlorite, the rock 
assumes a dull, dirty, greenish colour. 
(c) Coarse-grained Hemicrystalline Felspathic Rocks.—These rocks 
may show little trace of a base or groundmass, and will often puzzle the 
beginner, for it is the character of the finer grained matrix that in many 
cases differentiates one type of igneous rock from another. But with care 
and patience he may hope to distinguish most of those the essential 
components of which can be clearly seen with or without the aid of his 
pocket-lens. Very often the nature of the rock will be suggested by the 
aspect of its weathered crust. When this is light-coloured, and the rock 
has a white, greyish-white, or yellowish-white, earthy, chalky, or clay-like 
appearance, he may suspect that he is dealing with a more or less acidic 
rock. Should it be a guartz-forphyry or a rhyolize, it will show either 
granules or crystals of quartz, which may be readily separated from the 
decomposed matrix in which they are embedded. If, on the other hand, 
no trace of quartz be visible, then the rock will probably be orthoclase- 
porphyry, trachyte, phonolite, or andestte. In the case of andesites, how- 
ever, it must be remembered that the weathered crust is not infrequently 
rather brown and rusty. Coarse-grained éasal¢s, like the finer grained 
and compact varieties, are in like manner often recognisable by their 
conspicuous dark rusty brown or yellowish crusts. Removing the 
weathered crust, the character of which may have suggested the type 
of rock he is examining, the observer carefully scrutinises the component 
minerals in the usual way. If it be a basalt-rock, it ought to show on 
a fresh fracture an aggregate of striated felspars—numerous glassy-like 
rods—with entangled crystals and crystalline granules of dark or black 
minerals (augite and magnetite), and frequently scattered granules of a 
greenish mineral (olivine). An ordinary coarsely crystalline andestte 
will consist almost exclusively of laths or rods of a similar felspar, olivine 
being absent, and dark ferromagnesian minerals either apparently wanting, 
or, if present, not nearly so numerous as in a basalt-rock. When such 
minerals abound, then, without careful microscopic examination, it would 
be impossible to say whether the rock was basaltic or andesitic. The 
trachytes and rhyolites,as we have learned, are usually lighter coloured 
rocks than the andesites and basalts. Both are commonly distinguished 
by their rough feel. Their dominant felspar is orthoclase, of which the 
phanerocrystalline types appear to the naked eye to be almost entirely 
composed. Usually, however, we may detect small crystals of dark 
ferromagnesian minerals more or less sparingly entangled among the 
felspar crystals. Very often, too, large phenocrysts of glassy orthoclase 
(sanidine) are present. Should quartz be also present, then the rock is 
a rhyolite ; if it be wanting, we have a trachyte. Decomposing phonolite 
can hardly be distinguished from weathered trachyte. But the coarser 
grained kinds, on freshly fractured surfaces, usually show well-defined 
tabular crystals of glassy felspar, often arranged in parallel positions, and 
