BASIC VOLCANICS 123 



greater alteration of the feldspar. The feldspar is too thoroughly altered 

 for determination of species. The alteration product is for the most 

 part kaolin. Cloudy and granular epidote fills the interstices. Chlorite 

 and magnetite are also present. The structure is trachytic where not 

 obscured by secondary products. 



On Norfolk street, near Cooks court, occurs a great mass of these 

 lavas. The rock of this locality resembles closely that already described. 

 It is very fine grained and almost structureless. The original constit- 

 uents are more or less completely replaced by calcite, chlorite, epidote, 

 and quartz. The groundmass polarizes but faintly, and may have orig- 

 inally been, in part, glass. The feldspathic microliths are obscured in 

 outline and their species can not be determined. 



On Morton street near Cod man, in Dorchester, the basic volcanic 

 occurs as an aphanitic dark green rock, obscurely mottled with fine red 

 jasper areas, and with jasper deposited along the walls of cracks. The 

 slides show the same structureless or faintly ophitic groundmass, with 

 epidote, chlorite, quartz, leucoxene, and the iron oxides as constituents. 

 There are altered plagioclase phenocrysts. Hematite replaces completely 

 some phenocrysts resembling olivine in outline. 



On Delhi street, in Mattapan, occurs a very similar volcanic. It is like 

 the Morton street occurrence both in the hand specimen and in the slide. 



The alteration to chlorite and epidote is so far advanced as to disguise 

 the original constituents. One untwihned feldspar phenocryst whose 

 substance has not completely disappeared shows an extinction of 11° 

 The groundmass polarizes very faintly and seems to be composed of 

 altered orthoclase. There is considerable magnetite in the section and 

 some secondary quartz. 



A basic volcanic exposed in a high ledge on the west side of Central 

 avenue, Milton, is coarse grained and amygdaloidal. It also shows great 

 alteration. There are considerable areas of epidote, chlorite quartz, and 

 calcite. 



The structure in some cases is trachytic and porphyritic and the feld- 

 spar still comparatively fresh. Again, the extinctions on 010 and 001 

 and the maximum equal extinction angle indicate albite as the species. 



In this locality the rock exhibits a marked cleavage, parallel to which 

 it breaks in slabs about one-quarter of an inch in thickness. 



BASIC FRAOMENTALS 



Associated with the massive volcanics at Cooks Court and on Rock- 

 ville street near Blue Hill avenue are tuffs. At Cooks court the tuffa- 

 ceous character shows on the weathered surface. The rock is comopsed 



XVII— Bum-. Geoi,. Soc. Am., Voi,. 11, 1899 



