12 1 F. BASCOM — VOLCANICS OF NEPONSET VALLEY 



of angular fragments 'of plagioclase, quartz, orthoclase, of basic, and of 



acid lava. 



At Rockville street there occurs a dark purple tuff. Jt is composed of 

 heterogeneous fragments of basic volcanic, porphyritic or non-porphyrjtic 

 or amygdaloidal, a little of the aporhyolite, and some jasper. These 

 fragments are contained in an exceedingly fine grained silicious cement- 

 The tuff is completely altered. Chlorite, epidote, calcite, hematite, and 

 magnetite are the secondary constituents. 



CHEMICAL CONSTITUTION AND NAME 



The following is an analysis * of the basic volcanic of the Neponset 

 Valley basin : 



Si0 2 53. 75 



A1 2 3 18.37 



Fe ? 3 8.28 



CaO 3.22 



MgO 5.63 



Na 2 7.05 



K 2 1.20 



Loss by ignition ] Hz ° \ 3.34 



J \ Co 2 J 



Traceof P 2 5 1 Very slight 



MnO and C0 2 ' J 



Total 100.84 



The analysis shows a remarkably high soda content. This is so ab- 

 normal that the correctness of the analysis was called in question. A 

 new analysis was made, but the result showed no alteration in the soda 

 content. In the slides there is no indication of the presence of nephelite 

 or analcite. Whether the pyroxene or amphibole, which may have been 

 original constituents of the rock, was a soda pyroxene (aegerine) or a soda 

 amphibole (glaucophane or riebeckite), there is no absolute means of 

 determining, but there is no indication that this was the case. If there- 

 fore all the soda present is combined with silica to form feldspar, this 

 feldspar will constitute 59.75 per cent of the rock, while orthoclase con. 

 stitutes 7.11 per cent, and the other constituents 33.14. 



That some proportion of the soda was consumed in a magnesian silicate 

 is not improbable, because of the low silica percentage. If the soda and 

 potash are used exclusively to form feldspar, sufficient silica remains to 

 combine only with 2.66 per cent of the lime. Some of the lime may, 



♦Analysis made by Dr W. H. Walker, of the Massachusetts Institute of Technology, from com- 

 posite material carefully collected for the purpose. 



