194 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
the conglomerate, or else decomposed pebbles of felsite or petrosilex 
(ibid., p. 189-190). The melaphyr or amygdaloid he regarded as 
largely of sedimentary origin and as underlying the conglomerate. 
According to his view the rocks of the basin were thrown into a series 
of anticlinal and synclinal folds with numerous faults, the whole 
pitching eastward; the conglomerate became coarser toward the base 
and the underlying amygdaloid was exposed in the axes of the syn- 
clinals. 
Later Views. With the discovery of Wadsworth that the slates of 
Braintree are cut by granite it became necessary to regard the conglom- 
erate as more recent than some, at least, of the slate, for pebbles of 
granite occur in the conglomerate. The contention of Crosby that 
the conglomerate underlies all of the slate could no longer be sus- 
tained. In his later papers he still maintains that the conglomerate 
passes conformably upward into slate, but that there are two slates 
of different ages. With this view it may be said that most geologists 
agree. The occurrence of slate pebbles in the conglomerate is now 
admitted by Crosby. The work of Benton and others has shown 
that the amygdaloid is a true igneous rock and cannot be regarded as 
sedimentary. Moreover, it has been shown by Burr (b, p. 64) that 
the position of the amygdaloid cannot be regarded as inferior to the 
conglomerate but that it occurs at varying horizons and is not confined 
to anticlinal axes. ‘The main structural features of the basin as out- 
lined by Crosby in his earlier report are still believed to be correct 
but certain modifications have been necessary, which will appear in 
the more detailed account of the structure that is to follow. The 
southern and southeastern portions of the basin have more recently 
been worked out in considerable detail by Crosby and his results have 
been largely employed in the present discussion of those areas. 
General Structure:-- The Northern Boundary. The relative straight- 
ness and uniformity of the escarpment from Malden to Waltham has 
already been mentioned. In only one place is there an important 
interruption. At Medford a small area of conglomerate lies north of 
the Somerville slate and rests against the northern crystallines. The 
escarpment weakens at this point. The relations of this conglomerate 
to the neighboring slates are somewhat uncertain, because no satisfac- 
tory dip has been obtained. According to Crosby the conglomerate 
has a southerly dip and is overlaid conformably by quartzite or sand- 
stone. This seems to be the general view (Crosby, g, p. 16; LaForge, 
p. 74-76). Thus the conglomerate would appear to pass beneath the 
slates and to be continuous with the conglomerate south of the Charles 
