416 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



20. In the northeastern corner of the Pahner quadrangle a dike of the 

 same fine-grained, dark-gray diabase was discovered by my assistant, Mr. 

 C. S. Merrick. It is nearly a mile west of the point where the Boston and 

 Albany Railroad leaves the quadrangle, and appears in the crest of the hill 

 at the 900-foot contour.^ It is about 100 feet wide and strikes N. 20° E., 

 and is plainly a part of the dike No. 10, which can he traced north across 

 Ware and New Braintree. 



A MICROSCOPIC DIABASE DIKE FROM PELHAM, AND OLIVINE AND GLASS- 

 BEARING DIKES FROM MONSON. 



The two great diabase masses of tlie Triassic in Massachusetts, the 

 Deei-field and the Holvoke dikes, are amvgdaloidal at surface and aphanitic 

 at base, but everywhere normally crystalline, and everywhere, even when 

 seeming quite fresh, much decomposed. The series of smaller dikes of the 

 same rock, when run in the gneiss, parallel to and a few miles distant from 

 the eastern border of the sandstone, which were traced across Connecticut and 

 Massachusetts b}" Percival and Hitchcock, are in texture exactly similar to 

 the former, showing a typical diabase texture, but always very much 

 fresher. They often send off a great number of apophyses, which sink to 

 very small dimensions and run out in all directions and to considerable 

 distances through the gneiss, which, ordinarily very friable, is here so 

 indurated that thin flakes can be broken off and slides prepared containing 

 one or more of these minute dikes. An interesting slide of this character 

 from Pelham contains a dike 0.9™™ wide and 20™™ long. It is a tachylyte, 

 shading from dark gray at one side to jet black at the other, and under the 

 microscope is a colorless glass loaded with a fine dust, apparently magnetite. 

 The shading into black is due to the occuiTcnce of this material in much 

 greater quantity at one side of the dike, as if it had been formed horizontally 

 and the magnetite had sunk to the bottom. The rest of the surface has a 

 mottled look, like a miniature representation of a tiger's skin. This comes 

 from the fact that minute angular fragments of quartz and feldspar, which 

 are scattered through the mass, are surrounded by a halo of the same black 

 dust, outside which a broad ring of the glass is comparatively clear. This 

 gives the whole an apparent spherulitic structure, and this structure is really 

 present and the glass is in a state of tension around the foreign gi'ains, as 



'Percival, Geol. Conn., map. 



