44 ELEMENTARY GEOLOGY 
cite, but is somewhat more resistant, and is not made 
to effervesce by cold acid, although warm acids cause 
this phenomenon. It forms great beds of magnesian 
or dolomitic limestone, in which it is closely associated 
with calcite. 
Another form resembling calcite is the carbonate of 
iron, siderite (FeCO;), which, in some cases, seems to be 
BiG; 7 
Black mica. A thin cleavage piece above, is so transparent that printing beneath 
it may be read. 
calcite whose calcium has been replaced by iron. It 
is commonly a brownish mineral, looking like calcite, 
though somewhat heavier; and, although found in 
veins that can be worked for iron, it is not one of 
the important minerals of the crust. 
