
66 STRUCTURAL AND FIELD GEOLOGY 
limestones. Few limestones are without some proportion of magnesia, so 
that it is not possible to draw a hard-and-fast line between common 
limestone and magnesian limestone. It is only when the magnesia forms 
20 per cent. or so of the rock that the latter is included among the 
dolomites or magnesian limestones—but between this and limestones 
which contain little or no magnesia, there are all gradations. 
Rock-salt is a crystalline, fibrous, or even granular 
aggregate of sodium chloride, which occurs either in thin 
layers or in massive beds, sometimes reaching a thickness of 
several hundred feet. When pure it is clear and colourless, 
but is often stained with impurities, being frequently red, 
yellow, or grey. Blue and green tints also occur, but they 
are not common. The mineral is frequently turbid owing 
to admixture with sandy or argillaceous matter. In many 
places, indeed, it passes into a saliferous clay. It is usually 
associated with gypsum, anhydrite, and dolomite, interstrati- 
fied with clay, marl, and red or particoloured sandstones, and 
has obviously been deposited in salt lakes or in arms of the 
sea more or less cut off from the general body of open water. 
Gypsum, hydrous calcium-sulphate, occurs in beds, layers, 
or lenticular sheets and masses, and is often associated with 
rock-salt, anhydrite (calcium sulphate), dolomite, red clay, 
and sandstone, etc. In structure it varies from compact to 
granular, or it may be a fibrous, scaly, or sparry aggregate. 
When relatively pure it is white or quite colourless, but is 
often stained yellow or red by iron-oxides, or coloured grey 
and brown, owing to admixture with clay or other impurities. 
The exceedingly fine-grained varietics are known as Alabaster. 
Compact gypsum is readily distinguished from limestone, 
which it sometimes resembles, for it is scratched by the finger 
nail, while limestone 1s not. 
Siliceous Sinter is an aggregate of amorphous silica con- 
taining a variable proportion of water. It may be loose, 
unconsolidated, and porous, or dense and compact, and often 
assumes stalactitic and stalagmitic forms. When free from 
impurities it is white, but as these are often present it may be 
stained various shades of yellow or red. It is formed by 
deposition from thermal springs, as the result of evaporation ; 
in some cases, however, depusition is partly due to the action 
of minute alga, which occasionally flourish in the hot pools 
of a geyser region. 


