Geology of Martha's Vineyard, 245 



iow. The ferruginous pebble beds are brown, or reddish, 

 sometimes a deep blood red, and they are generally ce- 

 mented by the oxide of iron, so as, in some instances, to 

 require a considerable blow of the hammer to separate the 

 fragments. This is particularly the case in the lower part 

 of these strata ; where the iron ore, which appears to be the 

 argillaceous, is sufficiently pure to be wrought, although 

 penetrated by pebbles throughout. Some of the clay beds 

 are nearly half made up of small plates of silver coloured 

 mica, intimately mixed wiih the clay, which appears to be 

 kaolin. In this clay, beneath the ferruginous pebble beds, 

 I found good specimens of well characterised lignite. It 

 consists of flattened trunks, or branches, several inches in 

 diameter, of a clove brown colour, retaining, very distinct- 

 ly, its longitudinal, fibrous structure: but the cross fracture 

 is conchoidal and shining, and the concentric rings are in- 

 visible. The bark is a mere line in thickness. It burns 

 without much difficulty, with considerable flame, and emits 

 a pungent rather unpleasant odour. It lies horizontal in 

 the bed of clay, and is one of the exogenites of A. Brongn- 

 iart, (vid. Vol. VII, No. 1, Journ. Sci. p. 178.) In other 

 beds of clay, small masses of lignite occur, some of which 

 exactly resemble common charcoal, and burn as freely. I 

 saw no other organic remains in these strata, except a sin- 

 gle shell, in the ferruginous sand, which I lost. 



Viewed on a general scale, the beds, above described, 

 are nearly horizontal. But numerous minor irregularities, 

 in the dip of the strata, occur in the cliff, which I examin- 

 ed. Indeed, instances may be seen of almost every possi- 

 ble degree of declination : in some places, the beds arch 

 upwards, and in others they arch downwards. Whether 

 this irregularity does not proceed from a partial sliding 

 down of large masses of the clifT, I could not determine ; 

 though inclined to believe it does not. 



The above description, it will be perceived, corresponds 

 in its general characters, to the European Plastic Clay 

 Formation ; and therefore the strata it embraces have been 

 thus denominated. But in order to establish their identity 

 with perfect certainty, a comparison must be instituted be- 

 tween the organic remains, occurring in each series of 

 strata. It is not therefore, without some doubt, that I have 

 denominated the formation, above described, the Plastir 



