THE NEWER CORES AND SHORT DIKES. 493 



tional abundance. The feldspai's are wholly decomposed and the rock is 

 full of spots of diabantite. 



2. In various sections cut from specimens taken at diflFerent distances 

 up to 3,300 feet from the edge of the dike no distinction could be observed, 

 but in one taken from very near the center the augites were in large, dis- 

 tinct crystals, very abundant, and plainly anterior to the feldspars. 



3. Slides taken from the south edge of the dike, where Elmers Brook 

 finally leaves the trap, showed a large development of the finely granular 

 groundmass (grains 0.005"™) so common in the I'ock of the tenth dike. 



4. Sections were cut from the long, narrow, eastward prolongation of 

 the dike where the Moody Comers wood road crosses it and at its inter- 

 section by the two roads next east. They resemble the type closely. The 

 augite is in the main subsequent to the feldspars, but is a little more dis- 

 tinctly individualized in long blades. Olivine changed to an olive-green 

 serpentine and distinct traces of the unaltered mineral occui- sparingly. 



5. In sections cut from the edge of the small apophyses sent off by 

 the main mass into the sandstone and exposed in Batterson's quarry, we get 

 additional proof that the larger feldspars are of earlier consolidation. These 

 porphyritic feldspars are of the common size, 1 to 2™™ across, and are asso- 

 ciated with deep-green, well-formed olivines in an extremely fine-grained 

 groundmass, so that it seems that they had already separated out in the 

 iiiagma before its injection into the narrow fissure in the sandstone, in which 

 it cooled so rapidly that the customary ophitic structure was not produced 

 but was replaced by the semicrystalline development described below. 



The main groundmass is a felted mass of finest fibers 0.0016™™ across, 

 quite possibly feldspar microlites, which are not rigidly straight, but wavy, 

 often beaded, and are clearly margarites ; generally, however, they polarize 

 distinctly. These fibers have a radiated arrangement, which gives the 

 whole groundmass a spherulitic structure. The fibers polarize sheafwise, 

 although they are not parallel. 



The presence of olivine in the fresh fine-grained diabase dikes in the 

 granite, and especially in the minute dikes I have described (p. 416), as 

 also its presence in the newer diabase of the volcanic plugs, particulai'ly in 

 that one which has been described as so full of quartz grains (p. 483), may 

 seem, when contrasted with the absence of olivines in the great Deei-field 

 and Holyoke beds, to indicate that the two former occurrences are to be 



