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GENERAL. STRUCTURE OF THE NANTASKET AREA. 
The main structural features are so clearly expressed on the 
map that this topic may be treated very summarily. Nantasket 
is a monoclinal area; and, although it has been completely 
shattered by numerous faults and dikes, the strata exhibit only 
low and little variable dips, being almost unique in this re- 
spect among the sediments of the Boston Basin. Along the 
northern margin, from Atlantie Hill to Rocky Neck, the pre- 
vailing dip is south-southeast 10? to 20?. But southward, or 
toward the granite border, the dip usually diminishes, and 
becomes more easterly, the beds immediately adjoining the 
granite being often horizontal or having a slight inclination 
obliquely away from the granite. These gently inclined beds, 
together with the granite floor on which they rest, are broken 
by longitudinal and transverse faults into a series of blocks. 
A glance at the map, on which the faults are represented by 
broken black lines, shows that these blocks are extremely vari- 
able in form and size; and it is somewhat surprising that the 
unequal up and down movements or jostling of these numerous 
earth-blocks could take place without greater disturbance of 
their bedding-planes. 
The principal longitudinal faults, such as those immediately 
bordering the granite and the profound fraeture which skirts 
the southern shore of Nantasket Harbor, traversing the entire 
district from Straits Pond to Weir River Bay, downthrow to 
the north; and the displacements are so great that, although 
the bedded rocks dip toward the granite, in receding from the 
granite northward we pass in general from older to newer 
strata. The continuity of the strata is interrupted so frequently 
and completely, both longitudinally and transversely, by the 
numerous faults; and the conglomerate is, in the main, 8O 
homogeneous and, the tuff so local; that we are obliged to 
depend very largely upon the lithological characters of the 
interbedded lavas in correlating adjacent ledges as well as the 
more widely separated parts of the field. “The sequence of the 
