DIABASE DIKES AND STOCKS IK THE GNEISS. 415 



14. A vertical dike of black, fiue-grained, horizontally jointed diabase 

 runs east and west through the great quarry at Monson. It is nowhere 

 more than 16 inches wide, and as it goes upward it has a curious warji to 

 the south. It sends off many small dikes, which are specially discussed in 

 the next section below (p. 416). 



15. Farther south in Monson, in the crest of the bluff west of S. Mac- 

 intosh's house, is a dike of similar rock, 50 feet wide, running N. 65^ K., 

 which can be traced for some distance in the face of the cliff, cutting the 

 amphibolite. 



16. Still farther south, on the east slope of Peaked Mountain, west of 

 the house of J. Bliss, jr., occurs a dike of about equal size and of similar 

 character. These dikes in Monson were already traced by Percival. 



Trap bowlders are very abundant along the western slope of the high 

 ground east of the central valley of Monson, from one end of the town to 

 the other. 



17. Another plug occurs just over the State line in Stafford. It runs 

 N. 10° E., is 60 rods long and 200 feet wide. It is high up on the east 

 sloj)e of the hill which lies across the brook west of where the Hampden- 

 Stafford road crosses the State line. The slides show a trace of decompo- 

 sition. The feldspars of first generation have broad bands with wavy 

 extinction fi-om strain; the second are very complex twins. 



18. A mile S. 10° W. of this, where the road from the State-line Pond 

 to Somers rises to the top of a high hill, another dike is exposed just south 

 of the road. The contact, in granite, is exposed on the west. The strike 

 is N. 40° E. It is 56 feet wide, 200 feet long, has steep slope on the north 

 and a swamp on the south. The sections show unusually fresh and sharjily 

 and regularly outlined plagioclase of only one generation. 



19. A third stock of trap occurs a half mile S. 10° W. of this, which 

 crosses the next east-west road. It is 45 rods long from north to south, 25 

 rods from east to west. The gneiss is contiimously exposed around its east, 

 noi-th, and west sides. It is a compact, light-gray trap. In these sections 

 the large plagioclase crystals of first consolidation have the central portion 

 out nearly to the border changed into a cottony mass of plumose, micalike, 

 elongate, ragged scales, while the clear border shows at one end a marked 

 wavy extinction and the other end extinguishes sharply at 25° on either 

 side of the twinning suture. This is an vmusual change to some micaceous 

 or zeolitic mineral, instead of to kaolin. 



