F— V 



CAPE COD GEOLOGY 



141 



were there more good exposures, but no further subdivisions can now be given 

 than those observed by Woodworth at Gay Head, namely: 



4. Marine sand and clay "of Upper Cretaceous age." 



3. White micaceous sand and clay. 



2. Noduled clay and leaf beds. 



1. Lignite beds. 



The thickness of the plant-bearing beds in the Gay Head section must be taken with 

 due allowance for thinning and thickening in close folds and for overthrusts, as well as for 

 the probable occurrence of the marine Cretaceous at the summit of the pre-Tertiary series. 

 One hundred and fifty feet is probably in excess for the beds exposed above sea level. ' 



This estimate includes only the contorted beds exposed in the Gay Head cliffs, 

 beneath which, according to the estimate of the depth of bed rock, there are 

 500 or 600 feet more of undisturbed beds. 



CHARACTER OF THE DEPOSITS 



The Cretaceous deposits are unconsolidated and consist largely of clay, 

 the largest constituent of which is kaolin. They vary slightly in composition 

 and greatly in color. The colors are due largely to the weathering of forms of 

 iron and other extraneous material in the clay. Many beds of red and brownish 

 clay would be bluish gray or light gray if unweathered. Some beds that con- 

 tain marcasite have assumed, on weathering, a bright yellow color, due to a 

 coating of alum. Some beds of the purer clay are almost white; some beds of 

 the impure clay, such as the lignitic beds, are black. All the beds of lignite 

 contain clay, but parts of some of them consist of fairly pure lignite. The exten- 

 sive white beds seen at Gay Head and elsewhere are composed of grains of 

 kaolin and quartz and scales of muscovite, a mixture suggesting transported 

 arkose in which the feldspar has been altered to kaolin and muscovite. 



All the deposits, and even the fossil plants, are very similar to those of 

 the Cretaceous age in New Jersey. 



SOURCE OF MATERIALS 



The nature of the materials composing the Cretaceous deposits on Marthas 

 Vineyard makes it possible to form a picture of the conditions there when they 

 were laid down and to determine, at least tentatively, the area from which 

 the materials were brought. 



The lowest Cretaceous subdivision except a small amount of lignite is nearly 



1 Woodworth, J. B., Unconformities of Marthas Vineyard and of Block Island, Bull. Geol. Soc. 

 America, 8, p. 200, 1897. 



