1891. J NEAV YORK ACADEMY OF SCIENCES. 15 



ite was coined by Eosenbusch/ and was first applied to a dike rock 

 of tracbytic habit that occurs with the elaeolite-syenite of Brazil. 

 The name is based on the descriptions of rocks of this character 

 from near Boston, under the names porphyry and trachyte by 

 Wadsworth, Diller, and others. These rocks from the Champlaiii 

 Valley hav^e a prevailiufjly light tint, which is usually a creamy or 

 brownish white, but which is also in instances a lio-ht chocolate. 

 They have a general rough and granular feel and fracture very like 

 typical trachyte. Very rarely this is lithoidal or half vitreous, like 

 a lithoidal obsidian. Phenocrysts are not in general specially 

 marked, the less so, because widespread alteration makes fresh ma- 

 terial difficult to obtain. When present they exhibit the shining 

 cleavage faces of feldspar. One dike from the east side of Shel- 

 burne Point, near Burlington, consists almost entirel}" of the large 

 porphjn-itic feldspars. (Quartz is much rarer, although recognized 

 as a phenocryst in two dikes. 



Under the microscope these rocks are seen at once to have a 

 marked and characteristic trachytic structure, by which is meant 

 that the ground mass consists of small feldspar rods, not infre- 

 quently in fluidal arrangement. Between the rods one can some- 

 times detect small masses of interstitial quartz. The ground mass 

 is invariably holocrystallino and the feldspars are idiomorphic. 

 The quartz is not. The phenocrysts average 3-5 mm. Although 

 generally kaolinized, it can be seen that they are but once twinned, 

 and are in the great majority of cases orthoclase. The small crys- 

 tals are also once twinned, but as shown by the analyses they are 

 probably both orthoclase and anorthoclase. A very few plagioclases 

 have been noted. When alteration has made the feldspars of the 

 ground mass muddy or kaolinized they look like so many sections 

 of grains of wheat lying together. 



It is a remarkable fact that in no case have any recognizable dark 

 silicates been found in a slide Spots of limonite and iron stains 

 are indeed seen in some, and these may have once been bisilicates, 

 but it is doubtful, for none appear in the freshest material. Nor is 

 any magnetite noted. A few stray crystals and masses of pyrite 

 alone appear. The dikes are singularly free from any basic min- 

 erals and present a very pure, crystallized, feldspathic magma. 



In alteration the dikes afford nests of calcite and quartz, and the 

 usual muddy kaolin is abundantly present. The calcium for the 

 formation of calcite was doubtless derived in large part from the 

 neighboring slates, in which this mineral is very common. 



The following analyses illustrate the range of compositions. No. 

 66 is probably nearest the average. While it appears very fresh 



1 M. Hunter and H. Rosenbusch. Ueber Moncbiquit, ein Camptonitiscbes 

 Gan<;-gestein aus der Gefolgscbaft der Elaeolitb-syenite. Tscliermak's Min. 

 u, Petr. Mitth. xi, p. 445, 1890. Tbe bostonite, is incidentally mentioned 

 evidently as a "caveat," bnt applies so well that we are glad to adopt it. 

 After our determinations bad been made. Professor Rosenbusch kindly looked 

 over some specimens sent him and pronounced the rocks to be typical bostonite. 



