THE NEWER CORES AND SHORT DIKES. 487 



still polarizes brilliantly, is surrounded by a broad layer of fibrous structure, 

 the fibers radiatiuy and wholly amorphous. 



Magnetite is rare and in small grains only. 



The groundmass is made up of angular and rounded grains 0.001- 

 0.005°"" across, which can at times be seen to be twins, and they seem to be, 

 in part at least, augite, as they show an extinction at 42°. A specimen 

 from the north edge of the dike has the lai"ge feldspars so filled by these 

 minute augites that they occupy the whole space as closely as they do in 

 the surrounding groundmass. Indeed, it appears as if a portion of the 

 groundmass having a regular crystalline outline had been preserved, intact 

 from all decomposition, so that the interstices of the grains have not been 

 filled with the fine dust of limonite, kaolin, etc., which renders the rest of 

 the ground clouded. With polarized light the grains are seen to be optic- 

 ally orientated in the feldspar, as they extinguish together, and the feldspar 

 bands can be distinctly seen shining through. The groundmass is for the 

 most part the same in the portions included in the large feldspar crystals as 

 outside, but some inclusions are red-brown and apparently glass. I can not 

 detect with certainty any glass in the groundmass itself. The inclusions 

 are plainly from granite: quartz with sheets of pores, some containing mov- 

 ing bubbles, and rutile needles, microcline, centrally decomposed albite with 

 extinction angle 4°, and orthoclase. Fragments of granite with feldspars 

 wholly altered, and of an amphibolite quite fresh and closely resembling the 

 fine-grained rock at the northeast corner of Amherst, also occur. 



A specimen from the first cleared field north of the brook, externally 

 like the last, shows both the feldspars and the augite perfectly fresh and 

 colorless, sharply defined, and distantly scattered in the ground. There is 

 so much hematite that it takes up a considerable portion of the surface. 

 One quartz inclusion is surrounded by a coloi'less I'adiated fibrous layer, and 

 outside this by a broad band of hematite. The hematite so often surrounds 

 the foreign inclusions in a rock otherwise fresli that one is tempted to 

 assume it to have been a cement covering the grains before then* envelop- 

 ment in the lava. 



A great number of bodies are present having exactly the shape of 

 olivine crystals and a bright-yellow or red color. The yellow scarcely 

 polarizes at all, some few fibers or isolated spots showing faint color, and 

 it seems to be a yellow serpentine pseudomorph. The red sIioavs a peculiar 



