88 ELEMENTARY GEOLOGY 
hundred feet thick. Certain limestones, gypsum beds, 
etc.,' are also formed as precipitates from water which 
is thus subjected to evaporation. 
Not only is this now happening in these places, but 
in past times it has occurred in many other regions 
(such as central Texas, Kansas, Michigan, west central 
New York, and elsewhere), where now the conditions 
of climate forbid. In these places layers of salt and 
gvypsum are found deeply buried in the earth, bedded 
with other strata. 
Resemblance to the other Rocks. Since these rocks are 
deposited in water, they are often crystalline, though 
this is by no means a necessary condition. These differ 
from most of the other crystalline rocks in the fact 
that they are made, not of several minerals combined, 
but of a single mineral, though this is often mixed 
with minor quantities of impurities. Moreover, they 
are deposited in layers and therefore are stratified, 
though in a single small specimen the distinct bands 
may not be visible. | 
There is no certain test by which the small specimen 
of limestone, which is precipitated by one of these 
agencies, can invariably be told from another that has 
been derived either by mechanical or organic action; and 
1 Some of these rocks are described under other headings ; but no descrip- 
tion of their characteristics is attempted, for it is believed that the only way 
of learning these is for each student to examine and observe from actual 
specimens. 
