SOSMAN AND MERWIN : DATA ON PALISADE DIABASE 391 



temperature for a measured time, the rock was quenched by 

 fusing off its supporting wire and dropping it out of the furnace 

 into a basin of mercury. Microscopic examination then showed 

 what changes had taken place at the high temperature. A fresh 

 fragment was of course used for each test. 



The lowest temperature at which tests were made was 751°. 

 After the diabase had been held at this temperature for fifteen 

 minutes, and then quenched in mercury, the phenocrysts of 

 feldspar and pyroxene showed no change whatever; in the augitic 

 groundmass a red brown mineral had formed, having an index 

 of refraction of 1.74 and resembling certain basaltic hornblendes 

 high in ferric iron. 



There were no indications of fusion at 751°, nor at 850°, 953°, 

 1035°, 1052°, and 1101°. Fragments of the rock were held fifteen 

 minutes at the first four of these temperatures, and seventy-five 

 minutes at the last two. All gave practically the same result as 

 the quenching at 751°, except that the red brown mineral de- 

 creased in amount with rising temperature. A dusty yellow- 

 brown material which could not be identified^, appeared in small 

 amount at 1101°. 



After seventy-five minutes at 1150° the phenocrysts were still 

 unchanged, but the clear red-brown hornblende (?) had disap- 

 peared. A dusty yellow-brown substance, too dark for the de- 

 termination of its optical properties, appeared to coat other min- 

 erals in films, and showed a few bubbles in places. Its occurrence 

 suggested strongly the beginning of fusion of the lowest melting 

 portion of the rock. Fifteen minutes at 1175° gave a little dark 

 glass which could be positively identified, so that we may take 

 1150° as the approximate beginning of the temperature-interval 

 of fusion. Feldspar and pyroxene phenocrysts remained un- 

 changed at 1175°. 



Fifteen minutes at 1200° produced considerable brownish bubbly 

 glass of refractive index 1.60-1.61, and fused nearly all the pyrox- 

 ene phenocrysts. Seventy-five minutes at the same temperature 

 practically removed the pyroxene and gave a more homogeneous 

 glass, none having refractive index above 1.60. The feldspar 

 phenocrysts, except around their borders, remained unchanged. 



