Acidic rock

Acidic rock is rock that is either siliceous, having a high content of silica (SiO2), or rock with a low pH. 
The two definitions are not equivalent, e.g., in the case of basalt, which is never high in pH (basic), but is low in SiO2. The term may be used in a third way, to refer to non-calcareous rocks, i.e., rocks testing negative for carbonate by not bubbling if a 10% solution of HCl is applied in a spot test.