354 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
feet in thickness, and contains well rounded pebbles of quartzite and 
of rhyolite. The source of these pebbles is uncertain. It seems 
very probable that the quartzite pebbles were derived from the 
quartzites now exposed in the northern part of the Middlesex Fells. 
The quartzites of the immediate vicinity are younger than the con¬ 
glomerate. The original rhyolite from which the rhyolite pebbles 
were derived was porphyritic, with phenocrysts of probably both 
orthoclase and j^lagioclase feldspar. The subsequent alteration, 
chiefly to kaolin, has been so great that the type of the phenocrysts 
can only be determined from the outlines of the crystals. The 
nature of the much decayed cement of the conglomerate is very 
obscure. 
East of the dike and in a position which would seem to bring it 
stratigraphically under the conglomerate, were the latter exposed 
east of the diabase dike, is a large outcrop of grayish pink arkose. 
Microscopically, this rock consists of very finely fragmental quartz 
grains with, apparently, an admixture of felsitic material. Epidote, 
zoisite, hematite, and magnetite are present in small amount. The 
base of the arkose is indeterminable. Some two hundred and fifty 
feet north of this outcrop is a small exposure of dull white quartzite 
whose relationship to the arkose is uncertain. Its position suggests 
that it is separated from the arkose by a bed of conglomerate 
similar to that exposed further west, an inference which is strength¬ 
ened by the known relative positions of similar series in West 
Medford. 
So far as the present available evidence goes, the breccia may 
tentatively be considered as of volcanic origin, and probably later 
than the sediments. The breccia seems to have been preceded, in 
the order indicated, by the formation of the arkose, the conglomer¬ 
ate, and the quartzite of the area. The source of the material is at 
present uncertain. The exposures of sediments in the area are too 
small to permit of the determination of their structural relations. 
The absence of pebbles of granite in the conglomerate, and the fact 
that the whole stratified series seems to dip towards the igneous 
rocks, would appear to indicate that the stratified rocks are older 
than the granite to the north. So far as has been determined, there 
is no definite evidence offered in this area as to the relative ages of 
the two. Diller (’ 81 , p. 173) states, however, that in an adjacent 
area the granite is found overlying the upturned edges of the strati¬ 
fied series. 
