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Silt is the material which forms the muddy bottoms and shores of bays and 
harbours. 
Limestones are composed mostly of carbonate of lime, and will effervesce with 
acid. They are compact, dull, of many different shades of colour, varying in 
structure from earth to semi-crystalline. (See Calcite.) 
Magnesian Limestone . — This contains a good deal of carbonate of magnesia, 
as well as lime, and is generally much harder than limestone, and does not effervesce 
as freely with acid. 
Chalk is a soft white earthy limestone, which will leave a mark on a board. 
Travertine is a massive, often nodular, limestone deposited by springs or 
water charged with lime. 
Crystalline. 
Crystalline limestones are limestones which have been altered, having a 
crystalline structure, and vary greatly in colour according to the impurities. 
Marble is a crystalline limestone. 
Dolomite cannot be distinguished by the eye from marble, but it contains a 
large proportion of carbonate of magnesia. 
Oranite consists of quartz, feldspar, and mica, but has no appearance of 
layers in the arrangement of the mica and other ingredients. The quartz is 
generally whitish, without any cleavage. 
The feldspar is generally whitish or flesh-coloured, and may at once be dis¬ 
tinguished from the quartz by its cleavage and surfaces, which reflect light 
brilliantly. 
The mica is in bright scales, either silvery, brownish black, or black, and can 
be split with a knife. 
Gneiss is a hard, compact rock, like granite, but the mica and other 
ingredients are more or less distinctly in layers. 
Syenite consists of quartz, feldspar, and hornblende. 
Mica Schist consists largely of mica arranged in layers, with usually some 
quartz and a little feldspar. This arrangement of the mica causes it to divide 
easily into layers. 
Hornblende Schist .—A dark greenish schistose rock, generally containing some 
quartz, but often composed almost entirely of hornblende, which is in layers. 
Chloritic Schist is of a dark green colour, with a greasy feel, and contains 
layers of chlorite. 
Mica Slate is a slate through which layers of mica run. 
Felsite consists mostly of feldspar, with a little fine quartz mixed through it, 
of a compact, fine, granular structure. 
Biorite is a greenish rock containing feldspar and hornblende. It always 
occurs in the form of dykes. 
Soapstone consists chiefly of talc of a brownish, greyish, or greenish colour, 
feels very soapy, and can be easily cut with a knife. 
Serpentine is of a dark greenish coloured rock of a waxy appearance, and can 
be scratched easily with a knife. 
Basalt is a black or greyish black rock with a tackey feel. It is of volcanic 
origin, occurring either as dykes or flows. It is often full of bubbles, which in 
some cases have been filled in with silica or calcite, when it is called an 
amygdaloid. 
