ty a Se ee 3 
Feta ss =: Bi: 
154 GHOGNOSY. — ~ [Book II. 
caleareous organic fragmental rocks (p. 107) actually undergo a 
gradual internal change which more or less effaces their detrital 
origin, and gives them such a crystalline character as to entitle 
them to be ranked among the crystalline limestones (p. 112). 
1. Gravel and Sand Rocks (Psammites). 
As the deposits included in this subdivision are produced by the 
disintegration and removal of rocks by the action of the atmosphere, 
rain, rivers, frost, the sea, and other superficial agencies, they are 
mere mechanical accumulations, and necessarily vary indefinitely in 
composition, according to the nature of the sources from which they 
are derived. As arule they consist of the detritus of siliceous rocks, 
these being among the most durable materials. Quartz, in | 
particular, enters largely into the composition of sandy and gravelly 
detritus. Fragmentary materials tend to group themselves accord: 
ing to their size and relative density. Hence they are apt to occut 
in layers, and to show the characteristic stratefied arrangement of 
sedimentary rocks. They may enclose the remains of any plants or 
animals entombed on the same sea-floor, river-bed, or lake-bottom. 
Cliff-déebris. Moraine-stuff.—Angular rubbish disengaged by 
frost and ordinary atmospheric waste from cliffs, crags, and steep 
slopes. It slides down the declivities of hilly regions, and accumu- 
lates at their base, until washed away by rain or by brooks. It 
forms talus slopes of as much as 40°, though for short distances, if 
the blocks are large, the general angle of slope may be much steeper. 
Jt naturally depends for its composition upon the nature of the solid 
rocks from which it is derived. |The material constituting glacier 
moraines is of this kind; it may be deposited near its source or may 
be transported for many miles on the surface of the ice. 
Perched Blocks, Erratic Blocks.—Large masses of rock, often 
as big as a house, which have been transported by glacier-ice, and ’ 
have been lodged in a prominent position in glacier valleys or have 
been scattered over hills and plains. An examination of their minera- 
logical character leads to the identification of their source and, conse- 
quently, to the path taken by the transporting ice. (See Book III. 
Part IL. Section 11. § 5.) : 
Rain-wash.—A loam or earth which accumulates on the lower 
parts of slopes or at their base, and is due to the gradual descent of the 
finest particles of disintegrated rocks by the transporting action of 
rain. Brick-earth is the name given in the south-east of England 
to thick masses of such loam which are extensively used for making 
bricks. 
Soil.—The product of the subaerial dedomposition of rocks and. 
of the decay of plants and animals. Primarily the character of the 
soil is determined by that of the subsoil, of which indeed it is merely 
a further disintegration. The formation of soil is treated in 
Book III. Part II. Section ii. § i. 
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