158 CONTRIBUTIONS TO CHEMISTRY AND MINERALOGY. [bull. 167. 
In order to bring out the latter point more clearly, a series of rocks 
which had been analyzed in the laboratory of the United States 
Geological Survey was placed in a row of bottles and treated, just 
as the mineral species had been, with water and phenolphthalein. A 
granite and an amphibole-gabbro gave no alkaline reaction. A rhyolite, 
trachyte, leucite-basalt, feldspar-basalt, and diorite gave faint traces 
of color. Granitite, gneiss, phonolite, diabase, and camptonite yielded 
distinct alkaline colorations. 
In all of these instances the production of color is doubtless due to 
the solution from the mineral or rock of alkaline silicates. The note- 
worthy point is the quickness with which the reaction can be obtained. 
With minerals like cancrinite, sodalite, natrolite, pectolite, and 
apophyllite, the reaction is striking enough to be used as a lecture- 
table experiment. 
