Relations of the Igneous Rocks — Crosby. 41 
sidence introduced the deep water conditions permitting the 
deposition of the slate series into which the conglomerate 
series gradually merges upward. The deposition of the 
slate series was closed, it is supposed, by the Appalachian 
revolution, during which the Carboniferous sediments were 
strongly folded and faulted and injected by still more basic 
magma from, possibly, still greater depths, forming the 
older or east-west series of diabase dikes, now largely chlo- 
ritized or typical greenstone. Still later, and probably con- 
temporaneously with the Triassic sedimentation and ac- 
companying igneous activity in the Connecticut valley, 
were formed the diabase dikes of the newer or north-south 
series. 
With this the rock formations of the Neponset valley 
were complete, and its later geological history is recorded 
only in the erosion accomplished during later Mesozoic 
and Tertiary ages and culminating in the great ice invasion 
of post-Tertiary or Pleistocene time. 
THE CAMBRIAN STRATA. 
The existing small remnants, the larger less than a 
thousand feet long, of the body of Cambrian strata which 
we suppose to have once formed a continuous cover over 
the batholite in the Neponset valley, as in other parts of the 
Boston basin, appear to be confined to the vicinity of the 
Boston and Hyde Park boundary, in the eastern part of the 
Stony Brook reservation and the immediately contiguous 
territory. 
The sedimentary rock, of supposed Cambrian age, is all 
slate, of a uniformly massive, hard and distinctly meta- 
morphic character. The prevailing color is dark gray ; but 
it varies to lighter shades ; and very generally the rock is 
perceptibly veined or clouded with the green of epidote, 
indicating that the slate was. originally, more or less cal- 
careous, the lime having as an essential phase of the igne- 
ous matamorphism, combined with the alumina and silica 
of the slate to form epidote. This feature allies it with the 
Lower Cambrian slates of Weymouth, Quincy, Nahant, 
etc.; but in other respects it bears a striking resemblance to 
the massive, gray, non-calcareous Middle Cambrian slates. 
as these are developed on Hay ward creek in Braintree and 
