418 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



about a foot wide, I cat sections which showed, under the microscope, 

 many porphyritic ohviue crystals sharply outlined, some nearly fresh, but 

 most well advanced in the change to serpentine. Some were penetrated by 

 thick branching lobes of a brown glass, which in one case took up more 

 than half the surface of the section of the crj'stal and was accomjaauied by 

 two separate globules of the same glass with motionless bubbles. This is 

 the first certain occuirence of oli^'ine in the traps of the Connecticut Valley 

 in Massachusetts, and this, with that mentioned on page 411, the first occur- 

 rences of a glassy modification of the rock. The position of the olivines 

 and their large size suggest that they may have been formed at great 

 depths and fioated up to their present position. 



Another of the minute dikes, 2"" wide, in the gneiss from Monson was 

 cut. It had for part of its boundary a border of crushed gneiss, the triclinic 

 feldspar showing undulatory extinction, and the dike sent off" into this a 

 veinlet 0.1""™ wide. It was of finely granular, de^atrified tachylyte, with 

 a lighter border one-third millimeter wide. The feldspars in it were from 

 one-third to one thirty-eighth millimeter in length. The well-shaped oli- 

 vines allowed measurement of (021) A (021) = 98° (calculated 99° 06'). 



THE BEDDED OR COXTEMPORAISrEOUS ERUPTIVES. 



THE DEERFIELD SHEET. 



This, the most northern occurrence of eruptive rock in the Trias, 

 begins near the northeastern liorder of the latter, back of C. M. Conant's 

 house, in the west edge of the "sdllage of Grill, and extends west by south 

 past the house of J. Blake, where it is slightly faulted and where it has 

 a thickness of about 40 feet, which it maintains for a long distance. It is 

 compact at base and slightly porous at surface, and has low southeast dip 

 with the conglomerate in which it is intercalated. At its crossing of the 

 Gill-Turners Falls road it is again slightly faulted, and the bed was traced 

 only to this fault in my previous study of it.^ It turns here and runs down 

 to the mouth of Fall River, where it is again faulted. It is moved about 

 165 feet to the west and an opening made, through which the Fall River 

 reaches the Connecticut. From this point it runs down the west side of 



' The Deerfield dike and its minerals : Am. Jour. Sci., 3d series, Vol. XXIV, 1882, p. 195. 



