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Ree et 3 a Sma, 
Part Il. § vii.] DETERMINATION OF ROCKS. 179 
i. A fresh fracture shows the rock to be close-grained, dull, with no 
- distinct structure. 
a. H. 0°5 or less up to1; soft, crumbling or easily scratched with the 
knife, if not with the finger-nail; emits an earthy smell when 
breathed upon, does not effervesce with acid; is dark grey, 
brown, or blue, sometimes red, yellow, or even white = probably 
some clay rock, such as mudstone, massive shale, or fire-clay 
(p. 160); or a decomposed felspar rock like a close-grained 
felsite or orthoclase porphyry. If the rock is hard and fissile it 
may be shale or clay-slate (pp. 121, 160). 
f. H.1-5—2. Occurs in beds or veins (sometimes fibrous), white, 
yellow, or reddish. Sp. gr. 2°2—2:4. Does not eifervesce 
== probably gypsum (pp. 84, 115). 
y- Friable, crumbling, soils the fingers, white, or yellowish, brisk 
effervescence = chalk, marl, or some pulverulent form of lime- 
stone (pp. 111, 166). 
6. H. 3—4. Sp. gr. 2°5—2°7; pale to dark green or reddish, or with 
blotched and clouded mixtures of these colours. Streak white ; 
feels soapy ; no effervescence, splintery to subconchoidal fracture, 
edges subtranslucent. See serpentine (pp. 81, 152). 
e. H. averaging 3. Sp. gr. 2°6—2°8. White, but more frequently 
bluish-grey, also yellow, brown and black; streak white; gives 
brisk effervescence = some form of limestone (pp. 111-115, 165). 
é. H. 3:5—45. Sp. gr. 2°8—2°95. Yellowish, white, or pale brown. 
Powder slowly soluble in acid with feeble effervescence, which 
becomes brisker when the acid is applied to the powder of the 
stone. See dolomite (pp. 83, 114). 
y- H. 3—4. Sp. gr. 3—3:9. Dark brown to dull black, streak yellow 
to brown, feebly soluble in acid, which becomes yellow ; occurs 
in nodules or beds, usually with shale; weathers with brown 
or blood-red crust — brown iron-ore. See also clay ironstone, 
(pp. 84, 116, 175) ; and limonite (pp. 116, 174); if the rock is 
reddish and gives a cherry-red streak, see hematite (pp. 67, 
116). 
6. Sp. gr. 2°55. White, grey, yellowish, or bluish, rings under the 
hammer, frequently splits into thin plates, does not effervesce, 
weathered crust white and distinct— perhaps some compact 
variety of phonolite (p. 1389. See also porphyrite p. 144). 
u. Sp. gr. 2°9—3'2. Black or dark green, weathered crust yellow or 
brown = probably some close-grained variety of basalt (p. 147), 
or aphanite (p. 143). 
x. H. 6—6°5, but less according to decomposition. Sp. gr. 2°55—2°7. 
Can with difficulty be scratched with the knife when fresh ; 
White, bluish grey, yellow, lilac, brown, red ; white streak ; no 
effervescence = probably a felsitic rock (p. 136). 
A. H.7. Sp. gr. 2°5—2°9. The knife leaves a metallic streak of steel 
- upon the resisting surface. The rock is white, reddish, yellowish 
to brown or black, very finely granular or of a horny texture, 
gives no reaction with acid = probably silica in the form of a 
compact quartzite (p. 127), flint or calcedony (pp. 65, 117, 168). 
ii. A fresh fracture shows the rock to be glassy. 
Leaving out of account some glass-like but crystalline apne such 
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