BASAL BEDS. 139 



Absence of carbonaceous matter along the northern margin. Another factor which haS given 



free play to the interchanges of the iron oxides above mentioned in their 

 effect upon the color of the rocks is the very general absence of carbo- 

 naceous matter, or coal beds, in the basal series. In proportion to the 

 amount of carbonaceous remains present in the strata, the colors due to 

 the different states of iron are concealed. 



Extent of the arkose zone. — There is reason for believing- that the arkose bed 

 is nearly continuous about the margin of the basin, and that its disap- 

 pearance is due to faulting or to original local conditions which prevented 

 its accumulation. To what extent the sheet of arkose extends beneath the 

 basin is not known, as the rock is not exposed in the anticlinal axes in the 

 central part of the basin, for the reason that erosion has not there cut 

 down to the base. The arkose, together with the associated quartz con- 

 glomerates, wraps around the tongue of granitite and associated igneous 

 rocks which form the Wrentham-Hingham uplift between the Norfolk and 

 Carboniferous basins, and I see no reason why the Carboniferous basal 

 sediments should not have been continuous over this area, so as to unite the 

 two sets of beds in the present distinct basins. Until it can be shown, as 

 Crosby and Barton believed in 1880, that the arkose and higher strata on 

 either side of this granitic mass were derived from it alone, and not carried 

 over on it as a sheet of sediment from some still more northern area, there 

 seems no reason for accepting the view that this anticlinal ridge rose above 

 the general level in Carboniferous time. There are no general facts to show 

 that the arkose was carried along shore by currents more in one direction 

 than another. If anywhere thicker than at other points, the arkose is proba- 

 bly most developed in North Attleboro. In the eastern part of the basin, 

 and generally in the northern and eastern portions, there are many sand- 

 stones far above the base which would be denominated arkose, but they 

 are not to be mistaken for the mixture of quartz and feldspar of granitic 

 aspect which makes up the mass of the basal series. This arkose in some 

 instances differs but little from the original igneous rock whence it was 

 derived, except for the solution of the iron-bearing silicate, a slight tritura- 

 tion of the grains, and the intercalation of clearly stratified beds or occa- 

 sional waterworn quartz pebbles. 



