444 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



mica-like scales with their M (010) faces arranged parallel to the section 

 2:)laue, so that they show no twinning, but give with convergent jjolarized 

 light a negative bisectrix. 



In some of the large ca^aties a broad-bladed mineral, probably barite, 

 has formed in many separate and parallel plates, and all these have been 

 coated with albite and then removed by solution. Chalcopjn-ite also appears 

 in these cavities. There is no diabantite nor any trace of ordinary weather- 

 ing in the slide; and it is probable that heated waters acting on a magma 

 in which the first feldspars were floating have decomposed these, changed 

 all the iron into hematite, thus preventing the formation of the dark augite 

 and the black ores, and have then deposited the residuum of the feldspathic 

 material in the steam holes. Specimens can be obtained where the trap has 

 recently been blasted to make way for the electric road and the fragments 

 dumped on the steep slope extending down to the Deei-field River. 



The difference of the rock from the normal diabase is shown by order 

 of crystallization of the constituents of this and of the normal diabase. 



Diopside- diabase, Normal diabase. 



First plagioclase. Magnetite. 



Diopside. First plagioclase. 



Hematite. Second plagioclase. 



Second plagioclase. Augite. 



Steam boles. Steam holes. 



Third plagioclase. Diabantite. 



It is interesting to see here the development of the sericitic growth and 

 the albite mosaic without the formation of hornblende. 



The rock here incloses fragments of fine sandstone exactly like those 

 found at the Greenfield quany about 3 miles north on the same dike, and, 

 as there, it is greatly baked and fused with the trap. (See p. 419.) 



PAEAGENESIS OF SECONDAET MINERALS. 



During the summer of 1880 a heavy cut was made through the trap on 

 the south side of the Deei-field River for the extension of the Canal Raih-oad, 

 which opened up veins carrying the usual trap minerals in great abundance 

 and beauty. The veins run nearly vertical, with a thickness not above 4 

 inches, and they were exposed to a depth of 60 feet. Later a similar cutting 

 along the north side of the stream and directly opposite afforded many large 

 cavities filled with the finest transparent datoHte of unusual size, but lacking 



