106. Awencan Fern JOURNAL 
experience has taught the paleobotanist the horizons and 
kinds of rocks in which they are most likely to be found; 
but there is no general rule, for rocks heretofore barren 
may today be shown to contain them in abundance. 
It is simply a question of searching, and no likely rocks 
‘should be passed by. As ferns for the most part are 
delicate in structure, only certain kinds of rocks are fitted 
to preserve them in any degree of perfection. Thus, a 
coarse sandstone or conglomerate would not be likely to 
retain them so that the characters can be satisfactorily 
made out. Fine-grained clay shale is the matrix in _ 
oa detail of he nervation, and even the spore- 
_ €ases and spores. 
_ The ferns and fernlike planta o of the Carboniferous, or r 
: ‘the great anthracite coal age, are present in wide variety 
and perfection. ‘They grew in tropical profusion in and 
about t cet shallow, low-lying swamps of that time, 
“went only partial decay, the continued accumulation. 
: ultimately making 1 up the beds of coal. That the ferns 
_ were abundant i is shown by the fact that in numerous 
_ places layers of considerable thickness, compressed to 
the specific gravity : : tirely 
