158 . Naming Rocks. 
ing times of flood. Silt is the same material deposited 
in bays and harbors, where it forms the muddy bottoms 
and shores. Till is the unstratified sand, gravel and 
stones, with more or less clay, deposited by glaciers. ~ 
Called also unstratified drift. 
Detritus is a general term applied to earth, sand, 
alluvium, silt, gravel, because the material is derived, to 
a great extent, from the wear of rocks through disinte- 
grating agencies, mutual attrition in running water, and 
other methods (Dana). 
Clay—An exceedingly fine-grained, . soft, moist 
rock, formed of minute particles. It is the result of the 
decay of various aluminious silicates, always contain- 
ing water. When quite pure is is white, but generally 
colored red, blue, green gray, brown, etc., from the 
presence of various impurities. 
Mari—A general term used for all compounds of 
lime and clay. When clay predominates they are called 
clay marls; when lime is in excess, lime marls. They 
are compact rocks, breaking with a conchoidal fracture. 
They are of various color, from liver-brown and red 
red chiefly, and often contains nodules of limestone. 
M udstone—Massive, consolidated clay. It does not 
split into layers or lamine. 
Shale—A consolidated clay which splits into thin 
parallel lamin, which indicates various cessations and 
directions of the original deposition. Shale was prob-~ 
ably deposited as silt in the beds of rivers, lakes, estu- 
aries and seas. It is of various colors and shades, and 
often contains fossils. 
Slate—A hard, consolidated shale. It splits off 
into laminz, which have, however, nothing to do with 
