THE NEWER COEES AND SQORT DIKES. 483 



is only 5 or 6 rods in cross-section. The rock is black, fresh, compact, only 

 the larger and earlier generation of feldspars showing as fine lines with a 

 strong lens. The few small cavities show a white, compact, fibrous zeolite. 

 Under the microscope the rock shows the usual structure, with the 

 following peculiarities: The ai'ea surrounding the cavities is of much finer 

 grain than the remainder. The large feldspars are almost always made up 

 of only two broad individuals. An interstitial groundmass exists in consid- 

 erable quantity and decomposes to a red-brown mass. The cavities con- 

 tain (1) calcite, (2) diabantite, (3) wavy interstratifications of diabantite and 

 natrolite, (4) natrolite, (5) a red decomposition product of diabantite. The 

 whole slide is full of the finest feathery groups of magnetite octahedi-a. 



THE EIGHTH CORE. 



One will find this large and interesting mass by starting from the 

 southern boundary of the tuff, just across the brook south of the first 

 branching of the road after passing The Notch. The sandstone rises in a 

 low bluff facing north, and may be followed due east 100 rods across the 

 field and through the woods to a deep, di'y gorge running nortli. Follow- 

 ing this up across the tuff, one comes upon the trap at the head of the gorge 

 and can follow the line of contact very closely round in a great circle to 

 the point of starting, and it is plain in several places that the trap has come 

 up through the sandstone and is not a bed in it, and that it has come up 

 on the line between the sandstone and the tuff, intruding partly on the 

 area of the one and partly on that of the other. No amygdaloid was 

 observed, and the diabase is very fre.sh, dark-gray with shade of brown, 

 fine-grained, the earlier generation of feldspars being just \dsible to the 

 eye, and then- striation discernible with a strong lens. 



The induration of the tuff was very marked on its contact with the 

 diabase on the south side of the block. This would be the upper sui-face of 

 the bed if it had been a contemporaneous flow, and in this case there would 

 have been no trace of baking of the adjacent tuff. 



SOUTH HADLEY. 

 THE NINTH CORE; DIABASE WITH GRANITIC INCLUSIONS. 



An inspection of the map will show the next, or Black Rock, core to 

 consist of a great rounded mass which is situated south of the Holyoke 

 House and sends off to the east a long tail-like dike. If this be followed 



