MANSFIELD: ROXBURY CONGLOMERATE. 243 
it has been shown by Crosby that the felsitic floor has shared in the 
plications suffered by the overlying sediments (see page 202). 
Significance of Arkose. According to Daubrée, arkose is produced 
by the decomposition and disintegration of granite in place. The 
quartz separates out in small fragments which are angular, entirely 
irregular in form and without indication of crystal faces. The aspect 
of the arkoses of Bourgogne, Auvergne, and other regions, partakes 
of these characteristics. The quartz is angular and is mixed with a 
variable quantity of feldspar, more or less altered, and of mica. The 
rock is visibly the result of a simple rehandling of the granitic sands 
by water, without attrition (Daubrée, p. 255). The arkose at East 
Dedham, in the Boston Basin, and at Pondville, in the Norfolk Basin, 
corresponds very well with this description, while that in the Narra- 
gansett Basin is more decomposed and less easily recognized. 
Regarding the significance of arkose, all seem to agree that such an 
accumulation means a period in which granitic rocks were disinte- 
grated more rapidly than the loosened material could be removed, 
followed by a period in which the debris was hastily washed away 
and deposited. There is, however, some difference of opinion as to 
the climatic conditions under which the disintegration takes place. 
Shaler states his view in the following words: ‘‘ Judging by the condi- 
tions which have affected the fields that now afford or that might pro- 
duce the arkose deposits, we may assume that these levels of the Coal 
Measures time had long been the seat of a considerable rainfall and 
had maintained a coating of vegetation, such being the antecedent 
conditions of any decomposition that would prepare the way for 
arkoses” (Shaler et al., p. 52). On the other hand, Oldham, discussing 
the occurrence of undecomposed feldspar in sandstones of the Panchet 
group of India (one of the members of the Gondwana system), states 
that the disintegration of the parent rock, from which the materials 
were derived, went on at a greater rate than the chemical decomposi- 
tion of the constituent minerals. He thinks that this might be due 
either to extreme dryness, which would retard the rate of decomposi- 
tion, or to extreme severity of climate, which would accelerate the 
tate of disintegration (a, p. 201). 
The important feature of the arkose in this connection is the rela- 
tively fresh state of the contained feldspar. Under the conditions 
of considerable rainfall and vegetative covering, postulated by Shaler, 
it seems probable that the feldspars of the parent rock would undergo 
Comparatively speedy chemical disintegration, for water bearing vari- 
Sus mineral and organic solvents would percolate through all the 
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