650 



FRANK F. GROUT 



TABLE IV 

 Analyses of Amygdaloidal and Porphyritic Rocks 



1. Porphyritic diabase, Kettle River, east of Hinckley, Minn. Average of two 

 samples, some miles distant. F. F. Grout, analyst. 



2. Thomsonite-bearing rock. Good Harbor Bay, Lake Superior. C. F. Sidener, 

 analyst. 



3. Similar rock, decomposed, nearer Grand Marais, Lake Superior. C. F. 

 Sidener, analyst. 



1 



Amygdules and other secondary rock minerals. — Chlorite is prob- 

 ably the most important mineral of this group, forming amygdules, 

 pseudo-amygdules, and pseudomorphs after glass, olivine, augite, 

 and even plagioclase, with prehnite as an intermediate product. 

 The similarity of constituents of this green earth and augite suggests 

 that much of it originated from that mineral, and this idea is confirmed 

 by the absence of much chlorite in the rocks low in augite. The 

 variety of occurrence, however, as well as some small variations in 

 optical properties, indicates several methods of formation and a 

 variable composition. An early test of composition, by McFarlane 

 of the Canadian Geological Survey,' by fractional solution in nitric 

 acid and alkali gave a solution indicating the composition of delessite. 



I Canadian Geol. Survey, Report of Progress, 1863-66. 



