About 30 experiments, largely with feldspars, were made in HCl solutions at 300 degrees to 400 degrees C. (one at 525 degrees C.). The concentrations of K and Al were varied to some extent. It was found that kaolinite forms only below 350 degrees + or - 10 degrees C. and muscovite above this temperature to as high as 525 degrees C. Pyrophyllite may form over the whole range from 300 degrees to 525 degrees C. though it may be only metastable below 350 degrees C. K feldspars are metastable (probably also Na feldspars) at 400 degrees C. provided the concentrations of the corresponding alkalies are high. Some of these reactions take place at such low pressures that the presence of a liquid phase in the bombs is precluded. The writer has attempted to explain some of the reactions.