488 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



aggregate or patchy polarization, and seems to be a hematite pseudo- 

 moi*ph after ohAane. 



Another specimen from the -vacinity of the hist has a very different 

 stinictm-e. In a granuLir groundmass (0.01-0.02™™) there are regularly 

 disseminated, well-formed octahedi-a of magnetite, -sHisible with a lens in the 

 slide, and abundant diabantite-filled cavities. Small lath-shaped plagioclases 

 and augites are distantly scattered and inconspicuous. The rock resembles 

 that of the dike from the house south of the ruined leather mill below 

 Mount Tom station, on the west of the river. 



The above descriptions had been written before I received the first 

 accounts of Mr. Diller's discovery of quartz-basalt at the Cinder Cone, in 

 California. On sending him fragments of the rock here described, he wrote 

 that the qixartz resembled closely that of the Cinder Cone, and was more 

 abundant. 



It will be seen from my own descriptions that the idea that the quartz 

 was original in the rock had not occurred to me. It does certainly resem- 

 ble the Cinder Cone quartz very closely, and it is hard to see how a great 

 quantity of foreign sand could be included in an erupted dike, and espe- 

 cially how it could fail to liring with itself moisture enough to make 

 the rock vesicular. The shapes of the grains and the high greasy luster 

 are not like granite-quartz. I have not been able to verify my obser- 

 vation that the slides contain microcliue and mica, as the slides are not 

 now accessible, but the presence in the diabase, among many quartz 

 grains, of a large fragment (5"'" across) made up of quartz and orthoclase 

 is certain. The quartz was exactly the same rounded, bluish, greasy quartz 

 as the rest, and the flesh-colored feldspar gave the optical tests of orthoclase, 

 so that I feel quite certain that the unusual constituents have come in as 

 foreign inclusions. 



The structures produced by the introduction of this large amount of 

 foreign material into the liquid trap resemble those described from the 

 Greenfield bed in Chapter XIII (p. 419), where the sand has risen up into 

 this lava from below. This locality was studied many years ago, before 

 the Greenfield and Holyoke beds were understood, and I can not say what 

 modification of the above description might come from a new examination 

 of the place with new light. The l>lue color of the quartz may be due to 

 tension produced by heating, or the quartz may be derived from Algonkian 

 blue-quartz gneiss. (See page 29.) 



