HOLYOKEITE, A PURELY FELDSPATHIC DIABASE 

 FROM THE TRIAS OF MASSACHUSETTS. 1 



In the monograph of the three river counties in Massachu- 

 setts, 2 the writer described a "white trap" which occurs only in 

 scanty fragments in a bed of agglomerate interstratified in the 

 sandstone, a few feet above the surface of the great Holyoke 

 trap sheet at the east foot of Mount Tom, and a few rods north 

 of the station of the electric road going up onto the mountain. 

 The small angular fragments of the volcanic rock are scattered 

 rather distantly in the calcareous red sandstone, and seem 

 closely like a white, horny limestone spotted with chalcopyrite. 

 They include fragments of the coarse sandstone below the 

 Holyoke trap sheet, up through which they must have come, and 

 these inclusions are much coarser than the sandstone in which 

 they are included. The weathered surfaces show the rock to be 

 finely amygdaloidal, and acid brings out in the interior the same 

 structure which can indeed be seen, by attentive study with 

 a lens, on a freshly broken surface. A few grains of yellow ore 

 appear here and there in the body of the rock and in the round 

 cavities, and rarely the reflection of a twinned plagioclase lath 

 is visible. 



The thin section shows so exactly every structure of the dia- 

 base except those dependent upon the presence of iron in form 

 of magnetite and augite that one cannot help associating it 

 closely with the adjacent Holyoke sheet, as I have formerly 

 done by calling it a "white trap." Even the presence of much 

 chalcopyrite is characteristic. Under the microscope the texture 

 is exactly that of the trap of the large sheets minus the augite. 

 It is especially like the superficial portion of the Deerfield sheet 

 exposed at Cheapside. 



There is an ophitic network of very fine plagioclase needles 



1 Published by permission of the director of the United States Geological Survey. 



2 Monograph XXIX, U. S. Geol. Sztrv., 1898, pp. 365-474. 



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