THE HOLYOKE SHEET. 455 



the feldspars retaining their form. The araygdnles consist of radiated dia- 

 bantite, so fine-librous that it h^oks hke an ohve-green serpentine, and it 

 scarcely polarizes. In it are grains and crystals of calcite, and, floating 

 freely, man}- small feathery albite groups, visible only with the micro- 

 scope, and resembling those described (p. 443) from the cavities of the 

 Deerfield diabase. They are in twins; extinction 6 to 9° on each side 

 of twinning suture. In one case the angle of extinction with the trace of 

 OP (001) measured on oc P ob (010) was +4°, indicating albite. 



Also where the boundary of the trap swings farthest east at the south 

 foot of this peak the base of the trap is full of angular fragments of dove- 

 colored indurated clay, calcareous and having minute museovite scales, 

 and the two substances are molded together and the trap is amygdaloidal, 

 as at the occurrence on the south line of Holyoke. Under the microscope 

 secondary plagioclase rods like those in the diabantite cavities can be seen 

 in this rock, and they are visible as shining lines with a lens. The trap at 

 contact shows a distinct endomorphic change. The feldspar rods decrease 

 in size and number and the magnetite grains increase until a black opaque 

 border 2-3°"" wide intervenes. In other places the trap is shattered and its 

 fragments mingle with the sedimentary matter (see p. 368). A further 

 contact occurs just north of the Westfield-Holyoke highway, directly west 

 of the town line. Here the baking of the sandstone is marked, but the 

 trap above does not contain inclusions. 



NORMAL CONTACT OP THE SANDSTONE ON THE DIABASE. 



At all points where the upper contact could be seen the diabase is 

 very amygdaloidal and is often finely filled with secondary calcite and 

 zeolites. The sandstones rest upon the trap, filling irregularities, and not in 

 the smallest degree indurated or in any way showing heat effects. 



Dry Brook, which runs by Larrabee's quarry in the extreme north of 

 South Hadley, flows for a long distance west along the back of the south- 

 ward-sloping trap sheet, with sandstone for its lower (south) bank, and it 

 affords the longest continuous section of the contact. Following it up 

 eastward over the divide a similar valley runs east and continues to 

 expose the same contact, and farther east it is shown in each brook gorge 

 that comes down the south of the mountain. The amygdules are here 

 filled with natrolite and calcite, and form beautiful objects under the 

 microscope. 



