SEDIMENTARY AND METAMORPHIC ROCKS 81 
river floodplain, in the lake bottom, on the ocean floor, 
or in quiet bays along the shores of the ocean. These 
clay rocks are frequently so constructed that they 
readily split into layers. Such a rock is called a shale. 
Its habit of splitting depends upon the presence of 
many minute particles of flattish minerals, often mica. 
Near coral islands, the grinding action of the waves 
on the beaches wears the coral fragments into a fine 
clay. This settles on the bottom, forming a limy 
mud, which may afterwards become transformed to a 
limestone.' There are many other kinds of clay rocks, 
such as the kaolin clay, formed by the decay of feldspar ; 
or fire clay, which has lost its alkalies by the action of 
plants which grew upon it, and extracted these sub- 
stances for their own needs, leaving the clay so free 
of alkali that it resists the action of fire. Besides 
these, there are sandy, or arenaceous clays which con- 
tain considerable fine sand, carbonaceous clays, which 
contain abundant fragments of plants, etc. 
Among either of these three groups of fragmental 
rocks, there may be beds of volcanic origin; and as 
much of the volcanic ash and pumice floats into the 
sea, it may be gathered into layers, mixed with frag- 
ments from other sources. When a layer of rock is com- 
posed mainly of volcanic ee it 1s called a tuff. 
1This is but one of several ways in 2 which rar wea beds may be accu- 
mulated (see pp. 84 and 89). 
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