
ROCKS 83 
examination than is possible with the minerals locked together in a 
hand-specimen. A magnet drawn through the powdered rock will take 
up any magnetite that happens to be present. There should be no 
difficulty either in differentiating between minerals with good cleavage, 
and those which do not possess this property. Fragments of the former 
will be distinguished by their flat lustrous faces, while the latter will be 
quite irregular in shape. Should the rock be too fine-grained to allow 
of the rough separation of its constituents for examination with the lens, 
the powder will yet yield much information when studied with a low 
power under the microscope. A little should be placed on a glass slide, 
with a drop or two of water or oil added, and another glass-slip laid 
atop. By gently rubbing the upper slip over the powder, the grains 
can be still further reduced, and the individual constituents more 
thoroughly isolated. In this way the observer will often get evidence 
sufficient to enable him to pronounce on the true nature of the rock. All 
the common rock-forming minerals may be detected by this simple 
process just as readily as they can be by the examination of carefully 
prepared rock-slices. 
For purposes of determination in the field the more commonly occur- 
ring rocks (those, namely, which enter most largely into the formation of 
the earth’s crust) may be grouped under these four heads: 1. Argillaceous 
Rocks, 2. Calcareous Rocks, 3. Siliceous Rocks, and 4. Felspathic Rocks. 
Argillaceous and Calcareous Rocks are readily recognised. Their 
hardness ranges from less than I up to 3-5, and they are thus all readily 
scratched with a knife—many of them even with the finger-nail. A very 
soft rock, having a dull earthy aspect, and which when moistened is 
plastic, must be a clay. Should the rock be somewhat harder, and occur 
in thin irregular lamine, which may or may not cohere, it is an avgzé- 
laceous shale. If the shale, when rubbed down, be more or less gritty 
from the presence of grains of quartz, it is an avenaceous shale. Or, 
should it be black, and without gritty matter, it is most probably a 
carbonaceous or a bituminous shale, Or, again, if it be lighter in colour 
and effervesce with cold dilute hydrochloric acid, it is a calcareous shale. 
Clay-slate will be readily recognised by its structure—splitting, as it does, 
into firm plates along the superinduced planes of cleavage. Argillaceous 
rocks, when breathed upon, emit a peculiar earthy odour, and although 
this character is not confined to such rocks, nevertheless if a fresh rock, 
having this pronounced odour, be readily scratched, and present a dull 
earthy aspect, we may feel tolerably sure that it is argillaceous. 
Limestones are all easily scratched with a knife (the hardness being 
3 or less), and effervesce briskly with cold dilute acid. If the rock be 
relatively pure, it will weather with only a thin (generally yellowish or 
brownish) pellicle for a crust ; 1f it contains many impurities (clay, sand, 
iron-oxides), the weathered crust will be correspondingly thick. Dolomitic 
limestone is slightly harder (3 to 4) than common limestone. It effer- 
vesces very slowly with cold acid, but more briskly when the acid is 
heated. 
The Siliceous Rocks are distinguished especially by their hardness. 
