THE ARCHiEAN AGE. 
15 
becomes shale; if metamorphosed, it becomes slate. 
It can then be split in only one direction. This is 
due to what is called the slaty cleavage. If we ex¬ 
amine slate with a microscope, we shall find small 
micaceous fiakes arranged parallel to the lines of 
cleavage. If the metamorphism be continued, this 
rock becomes a schist; if there is much mica, it is a 
mica schist; if it contains considerable hornblende, 
or chlorid, or talc, it becomes a hornblende, chlorid, 
or talcose schist. If the minerals are arranged in 
bands, it may become gneiss. 
Sandstone is made up chiefiy of fine, small quartz 
grains, so hard that it will easily scratch glass. It is 
often colored by iron or other minerals; in the pres¬ 
ence of water the grains of quartz may become the 
centers of beautiful crystals. If other minerals are 
present to add color, the crystals may become agate, 
onyx, chalcedony, carnelian, amethysts, topaz, emer¬ 
alds, or other valuable gems. The crystals under 
other conditions may become augite, a very hard and 
usually dark stone. 
Limestone, under metamorphic agencies, becomes 
crystalline, and we call it marble; or if mixed with 
magnesium, it becomes dolomite, a coarse marble. 
Sometimes serpentine rock is mixed with it, and 
produces the beautiful, dark-green, veined marble. 
Sometimes iron gives it a cloudy brownish effect, as 
in Mexican onyx. 
Locks are frequently made up of fragments or 
pebbles bound together by one of Nature’s cements, 
usually mixed with sand. Such rocks are conglom- 
