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CAPE COD GEOLOGY 



all clay, some of which carries fossil leaves. This material must have come 

 from a land surface that was being elevated and subjected to erosion after a 

 long period of atmospheric weathering as a peneplain. The great quantity of 

 clay deposited must have been derived from feldspathic rock, such as granite 

 and other igneous rock, or from feldspathic sandstone or shale. If it was de- 

 rived from sandstone or shale the period of weathering was shorter. It might 

 even have been eroded from these rocks without weathering and later altered 

 to clay. If the materials are considered in connection with those of the next 

 higher deposits, it would seem that the weathering preceded the erosion, and 

 that they were not removed at once, as the gradient was too low to permit 

 their rapid transportation. 



The beds of kaolin-mica sand represent a continuation of the conditions 

 and processes favorable to the deposition of clay. They indicate that the relative 

 elevation of the land was rather rapid, so that the remaining products of alter- 

 ation were swept away quickly and laid down as cross-bedded deposits in 

 shallow water. The presence of quartz grains shows that the region under- 

 going erosion was originally composed of granitic rock. Such a region — that 

 is, one composed largely of granite that had been long subjected to weather- 

 ing and was being reduced to a peneplain — is found in New England to the 

 north and west of Marthas Vineyard. As to the origin of the lignite, however, 

 which forms the lowest Cretaceous beds exposed, two theories may be ad- 

 vanced. The first is that they were formed in large fresh-water or brackish- 

 water swamps from vegetation that grew and died and accumulated, without 

 much decay, to considerable depths in the places where they now lie. To this 

 vegetable matter a small amount of fine inorganic material was added by streams. 

 The second theory advanced to explain the origin of the lignite is that the 

 material now compositing it grew elsewhere and was brought to its present 

 site by streams and deposited in lagoons. Both theories have supporters, but 

 the first is favored here, although the beds contain no tree trunks or other like 

 forms such as we should expect to find if the lignite had been formed where 

 it is found, and all the fragments are considerably waterworn. Those who 

 adopt the second theory must hold that the lignite was brought to its present 

 site by lowland sluggish streams; otherwise more inorganic material would have 

 been brought in and mixed with the vegetable matter. The lignite must have 

 been deposited in large swamps or shallow lagoons in which organic material 

 collected, together with a small quantity of fine silt, which formed clay. 



