350 MKRRILL — DLSINTIiGUATlON AND DbX'OM POSITION OF DIABASE. 



Its inaximuin l)rea(Uh is stated to ])e not less than 500 feet, but it narrows 

 gradually northward to a diameter of not above 50 feet. A mile south 

 of the Mystic, on the same north-and-south line, is the so-called PoAvder 

 House dike of the same character, and presumably but a continuation 

 of the one at Medford. In point of age the dike is one of the youngest 

 of rocks of the vicinity, cutting the older eruptives, granite, " felsites " 

 and diorites, as well as the Carboniferous slates and conglomerates. 



Pp:trogiiapiiic Features of the Rock. 



The general petrographic features of the dike have been well described 

 bv Dr \\llliani H. llobbs, on whose paper* I shall draw for a descrip- 

 tion of the material in its least changed condition. The rock is quite 

 uniform in general character and for the most part sufficientl}^ coarse in 

 crystallization to permit the ready determination by the unaided eye of 

 its chief constituents, plagioclase feldspar, augite, biotite and occasional 

 ])yrite. Microscopical and chemical tests indicate the i)resence of two 

 feldsi)ars, the one laln'adorite and the second a more acid variety, pre- 

 sumably orthoclase. Original ai)atite, magnetite and ilmenite occur, and 

 secondary hornblende, chlorite, (piartz, calcite, leucoxene and pyrite. 

 The chemical composition of the rock is to be noted later. 



Earlier References to the Disixteciration. 



That this diabase had undergone extensive disintegration was sufh- 

 cientlv evident to have attracted the attention of the earlier geologists, 

 who indeed (M)uld scarcely fail to note so striking a feature in a region 

 which had been subject to extensive glaciation, and where as a conse- 

 quence only the small amount of residuary material from rock decom- 

 position that has been formed since glacial times is now to be found in 

 place. 



The first and most detailed description of this phenomenon I liave 

 been able to find is that given by J. F. and S. T.. Dana,t who state that — 



"At Powder House hill, in Ciiarleston, in the renter of ]\Ie(lford, in Reading 

 and in Wol)urn the <j;reenstone is most completely disinte<jrated, iind forms a 

 beautiful reddish l)rown sand which is much employed for forming hard gravel 

 walks. At these places the greenstone occurs in large globular masses, with a solid 

 nucleus surrounded hy concentric lamina of greenstone in various stages of decom- 

 ])osition. The lamina are of various thickness and are often easily separated. It 

 bears some rescml (lance to that variety of secondary greenstone called globular 



* Bull. Mus. Corap. Zoology, vol. xvi, no. 1, 1888. 



f Outlines of the Mineralogy and Geology of Boston and Vicinity, Mem. American Academy of 

 Arts and Sciences, vol. iv, 1818, p. 200. 



