Relations of the Igneous Rocks— Crosby. 35 
glomerate, some three miles in normal breadth, is, struc- 
turally, one simple, flat-topped and somewhat unsymmet- 
rical anticline, the central and dominant arch of the Boston 
basin (the Shawmut anticline), separated from the Blue 
hills or southern highlands by the Lower Neponset valley, 
and from the northern highlands by the Lower Charles 
valley, each of these main lateral valleys exhibiting, in the 
general view, a synclinal structure, with slate as the pre- 
vailing surface formation, but being, withal, as complex in 
geological structure as the central ridge or water-parting 
is simple. As thus defined, the Lower Neponset valley 
is, west of Boston harbor, a rectangular area some 
three miles wide and eight to ten miles long, including, on 
the mainland, small portions of the towns of Canton and 
Dedham, the whole of Hyde Park, the northern half of Mil- 
ton and Quincy and the southern half of West Roxbury 
and Dorchester. Jt is an area of great topographic as well 
as geologic complexity, and although, in general, low lying, 
includes, in Bellevue hill, the highest land within the Bos- 
ton basin. The district here included in the Neponset 
valley is not now wholly drained by the Neponset river, 
this study naturally following geologic, more closely than 
topographic or hyprographic, boundaries. 
The Lower Neponset valley is essentially an epitome 
of the entire basin, since it also consists of a central anti- 
cline of conglomerate bordered on either side by a well- 
defined slate syncline. The southern syncline, extending 
through Milton and Quincy, widens rapidly eastward, a 
somewhat open and composite trough, while the northern 
syncline. extending through West Roxbury and Dor- 
chester, is a relatively deep and narrow isocline. 
The immediate valley of the Neponset is developed in 
the complex and strongly denuded anticline which thus 
divides the more southerly of the two main troughs of the 
Boston basin, and which narrows eastward for the simple 
reason, that the axis pitches or inclines in that direction. 
The prevailing sedimentary rock of this belt is conglom- 
erate, and the attitude or structure of the conglomerate as 
a whole is anticlinal. It dips northward along the north- 
ern border, passing beneath the slate of the deep and nar- 
