242 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
and argues that this arrangement is due to the transmission of thrusts 
by lateral girders of compact crystalline rocks to the weaker rocks 
within the basin and that the effects of the thrusts are most marked 
at the borders because the more yielding sediments are not good 
transmitters (Shaler et al., p. 21-22). This argument appears to be 
well taken and seems to apply in the case of the Narragansett Basin 
but in the Boston Basin the evidence is not so clear. The thickness 
of the sediments in the latter region is, indeed, not nearly so great as 
in the Narragansett Basin but even so it would seem that similar 
phenomena should there be noted, even if in a less degree; but so far 
as the writer’s investigations have gone, there does not seem to be any 
definite tendency toward more marked metamorphism along the 
borders than within the basin. On the contrary, some of the best 
marked examples of deformation occur well within the basin. 
If the present sites of the various basins were originally separate 
erosion troughs or a single great depression, occupied either by rivers 
or by the sea, before the deposition of Carboniferous rocks began, it 
seems to the writer that there should be evidence of such conditions 
in the form of sedimentary deposits between the crystalline floor and 
the lower members of the Carboniferous series. Such rocks may 
indeed exist, overlapped by the Carboniferous beds, but so far as the 
contacts of the latter with the underlying terrane have been described 
no evidence of their existence has been found. In fact, there appears 
to be no definite evidence that any sediments were deposited in this 
region from Middle Cambrian to Carboniferous ‘times. It seems 
probable, therefore, that both the sites of the basins and the surround- 
ing country were land areas exposed to subaérial conditions prior to 
the deposition of the Carboniferous rocks. The fact that the conglom- 
erates in all the basins contain so large a proportion of granite pebbles, 
together with the fact that finer granitic debris is represented in the 
basal sediments of each basin, lends support to this view; for granite 
is ordinarily a deep-seated rock and its exposure at the surface at the 
beginning of Carboniferous deposition is in itself evidence of long 
continued existence as a land area before Carboniferous times. 
If the floors of the basins owe their present form to the orogenic 
agencies that folded the sediments it would be expected that they 
would show some effects of the deformation experienced both by 
themselves and by the overlying strata. Evidences of this character 
are not entirely lacking, for at certain places along the east side of the 
Narragansett Basin the marginal granite has been sheared and ren- 
dered locally schitose (Shaler et al., p. 122, 273). In the Boston Basin 
