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MANSFIELD: ROXBURY CONGLOMERATE. 245 
rocks originally red, the alteration of lime carbonate to siderite and 
then to limonite, as in the Devonian and Silurian rocks of the Appa- 
lachians, or to the decay of crystalline rocks containing iron (Shaler 
et al., p. 62). In the case before us there seems to be evidence of the 
agency of at least the first and last causes. Red strata of earlier date 
occur in proximity to the Carboniferous deposits, especially in the 
Narragansett Basin. But probably the decomposition of iron-bear- 
ing rocks, abundantly shown by the occurrence of arkose in all three 
basins, has been the most effective cause. According to Russell, 
red residual deposits are the product of warm humid regions, while 
the corresponding deposits of arid regions are not red but light colored, 
usually gray or yellow brown (a, p. 46). 
Precarbonijerous Climate. The abundance of the red rocks among 
the Carboniferous sediments in this region, therefore, seems at first 
‘Sight to indicate that prior to the deposition of the strata a warm and 
humid climate, perhaps similar to the tropics of the present day, pre- 
vailed in this region, as postulated by Crosby and Bouvé (Crosby, 
n, p. 463-464). But it has been shown that the arkose deposits, 
With relatively fresh feldspar, indicate that the moisture could not 
have been excessive. The climatic conditions could not therefore 
have been so humid as those of the present tropical regions, nor so 
dry as the arid tracts of the West. The combination of the arkose 
‘deposits with the red sediments must mean former climatic condi- 
tions of moderate or scanty rainfall and cool temperature. 
Since the great depth of disintegration shown by the character of 
the basal Carboniferous sediments indicates generally low grades 
Prior to the deposition of the strata, the cool temperature may be 
ascribed to general climatic conditions and not to altitude; for it is 
hard to reconcile low. grades of streams and deep disintegration of 
granitic rocks with high altitudes. The scantiness of the rainfall 
may be ascribed to the presence of mountains or a land mass, separat- 
Ing the region from the ocean on’ the east, more or less subdued but 
sufficiently high or extensive to intercept much of the rainfall. The 
Seneral character of the country in Precarboniferous times may per- 
haps be fairly compared to that of the present Piedmont plateau of 
the eastern United States, if the features of cool climate and country 
to the east were added, as above suggested. 
The supposition of cool climate for this latitude at the time the 
disintegration of the rocks was in progress is not wholly without 
ai fy for it has been shown on page 130 that actual glacial condi- 
i 
Ons occurred over an extensive region in the low latitudes of the 
