
te Ce Re Gee A 
5 Ry Sitoey eo Car ee 
180 3 GEOGNOSY. —*- [Boox I. 
-a8 quartz and rock-salt, the number of viireous rocks is comparatively — 
small. The true nature of the mass in question will probably not be 
difficult to determine. It must be one of the volcanic rocks (p. 104). If 
it occurs in association with sanidine or siliceous lavas (liparites, 
trachytes) it will probably be obsidian (p. 140), or it may be pitchstone 
(p. 140); if it passes into one of the basalt-rocks, as so commonly 
happens along the edges of dykes and intrusive sheets, it is a glassy — 
form of basalt (tachylite, hyalomelan, p. 149). 7 
iii. A fresh structure shows the rock to be crystalline. 
If the component crystals are sufficiently large for determination in 
the field, the name of the rock will readily be found. Where, however, 
they are too minute for identification even with a good lens, the observer 
may require to submit the rock to more precise investigation at home, 
before its true character can be ascertained. For the purposes of field- 
work, however, the following points should be noted. 
a. The rock can be easily scratched with the knife. 
(a) Effervesces briskly with acid = limestone. 
(b) Powder of streak effervesces less briskly. See dolomite 
(pp. 83, 114). 
(c) No effervescence with acid; may be granular crystalline 
gypsum (alabaster), or anhydrite (pp. 84, 115). , 
B. The rock is not easily scratched. It is almost certainly a silicate. — 
Its character should be sought among the massive crystalline 
rocks (p. 129). If, for instance, it be heavy, appear to be com- 
posed of only one mineral, and have a marked greenish tint, 
it may be hornblende rock (p. 121); if it consist of some 
white mineral (felspar) and a green mineral which gives it a 
distinct green colour, while the weathered crust shows more 
or less distinct effervescence, it may be a fine-grained diorite 
(p. 143), or diabase (p. 145); if it be grey and granular, with 
striated felspars and dark crystals (augite and magnetite), with 
a yellowish or brownish weathered crust, it is probably a 
dolerite (p. 148); if it be compact, finely-crystalline, scratched — 
with difficulty, showing crystals of orthoclase, and with a 
bleached argillaceous weathered crust, it is probably an or-. 
thoclase-porphyry (p. 138), or quartz-porphyry (p. 135). The © 
occurrence of distinct blebs or crystals of quartz in the fresh 
fractures or weathered face will suggest a place for the rock in 
the quartziferous cystalline series. 
iv. A fresh fracture shows the rock to have a foliated structure. 
The foliated rocks are for the most part easily recognizable by the 
prominence of their component minerals; their characters have been 
given at p. 118. Where the minerals are so intimately mingled as not 
to be separable by the use of the lens, the following hints may be 
of service :— 
a. The rock has an unctuous feel, and is easily scratched. It may 
be tale-schist (p. 120), chlorite-schist (p. 121), hydrous mica- 
schist (p. 123), or foliated serpentine (p. 152). 
B. The rock emits an earthy smell when breathed on, is harder than 
those included in a, is fine-grained and usually dark grey in 
colour, splits with a slaty fracture, and contains commonly 
scattered crystals of iron pyrites or some other mineral. It is 
some argillaceous-schist or clay-slate, the varieties of which 
