1891. } NEW YORK ACADEMY OF SCIENCES. bod 
ite was coined by Rosenbusch,' and was first applied to a dike rock 
of trachytic 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 Champlain 
Valley have a prevailingly light tint, which is usually a creamy or 
brownish white, but which is also in instances a light 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 entirely of the large 
porphyritic 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 holocrystalline and the feldspars are ‘idiomorphie. 
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 e 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. W hile it appears very fresh 
1M. Hunter and H. Rosenbusch. Ueber Monchiquit, ein Camptonitisches 
Gang-gestein aus der Gefolgschaft der Elaeolith- -syenite. Tschermak’s Min. 
u. Petr. Mitth. xi, p. 445, "1890. The bostonite is incidentally mentioned 
evidently as a “ caveat,” but applies so well that we are glad to adopt it. 
After our determinations had been made, Professor Rosenbusch kindly looked 
over some specimens sent himand pronounced the rocks to be typical bostonite. 
