PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 147 



stratification. This material was first taken to be included Cambrian slate but 

 proves on microscopic examination to be consolidated volcanic ash; compara- 

 tively fresh angular microscopic fragments of quartz and feldspar occur asso- 

 ciated with a micro-crystalline matrix; considerable sericite has been developed. 



The alkalies were determined as follows: Na 2 0, 3.85 per cent., K 2 0, 5.40 per 

 cent. The total alkali percentage is about that of the lava, but the proportions 

 are very nearly reversed. This is explained by the fact that soda silicate is 

 acted upon more readily by solvents and replacing agents than is potassa; the 

 development of sericite is evidence that the potassa practically remains in the 

 rock; this tuff, like all the fragmentals, yields more readily to alteration than do 

 the massive lavas; hence the abstraction of soda is not anomalous. 



Aporhyolite Dikes. — The rhyolitic dikes which cut irregularly the normal gran- 

 ite and its peripheral facies are cryptocrystalline, dense, commonly but incon- 

 spicuously porphyritic, and banded by flow movement; they are either irregularly 

 jointed or more commonly cleave parallel to fluidal lines; the less epidotized 

 material is brittle and breaks with sharp edges; rarely a spherulitic fabric can 

 be detected in the hand specimen. 



The large dikes are usually dark red or purple in the central portion of the 

 dike and greenish gray along the borders ; the small dikes are usually gray through- 

 out. Professor Crosby suggests that the rock was originally gray, that it was 

 subsequently reddened by oxidation and later bleached by deoxidation along the 

 borders through organic acids carried in the filtrating meteoric waters. A study 

 of the sections shows iron oxide in the form of hematite distributed through the 

 dark red material and granular epidote and sericite distributed through the 

 greenish gray material. A comparison of the analyses of material from the 

 center and from the edge of the dike shows a higher percentage of hygroscopic 

 water in the material from the edge; the addition of water is essential to the 

 production of epidote and sericite while the deoxidized ferric oxide (hematite) 

 may have been utilized in the production of epidote. The red material, as has 

 been stated, is free from hydration products. 



The original constituents are quartz, feldspar, iron oxide, and a scanty fer- 

 romagnesian constituent, the original nature of which is somewhat in doubt, but 

 which was probably biotite. The secondary constituents are sericite, epidote, 

 chlorite, and hematite. 



Feldspar occurs in two species and in two generations; the phenocrysts are 

 an alkali feldspar, possessing about the composition AbgAm and an orthoclase ; 

 m the groundmass both orthoclase and an albite of a more acid composition 

 occur (sometimes Ab). Quartz occurs only in the groundmass. The fabric of 

 the groundmass is either micropoikilitic or orthophyric or spherulitic; with the 

 first named fabric the feldspar is always narrowly lath-shaped, and the quartz 

 occurs in irregular areas including the other constituents ; with the orthophyric 

 fabric the quartz fills the interstices; in the case of the last named fabric the 

 spheruhtes do not possess regular and definite boundaries but a radial arrange- 



