South-Western Part of Somersetshire. 343 



instances prevails to the entire exclusion of any other ingredient, 

 forming a granular quartz rock : it is more abundant in the aggre- 

 gates of a coarse grain, clay being the chief ingredient in those of a 

 close and fissile texture. They have all an internal stratified struc- 

 ture, which is less apparent in those of a coarse grain, (and in a ca- 

 binet specimen scarcely discernible) but which becomes gradually 

 more distinct as the texture becomes finer, and at last the rock 

 graduates into a fine grained slate, divisible into laminse as thin as 

 paper, and having the smooth silky feel and shining surface of the 

 clay slate of a primary country. Alternations of the fined grained 

 slaty varieties with those of the coarsest structure in many successive 

 strata and without any regularity of position, are of constant occur- 

 rence, and frequently without any gradation from one structure into 

 the other.* In some instances portions of slate are contained in 

 the coarse grained varieties. Scales of mica are frequent, and they 

 all contain oxide of iron in greater or less proportion, and to the 

 different states of this oxide their various colours are, 'no doubt, to 

 be ascribed. The prevailing colours are reddish brown and green- 

 ish grey, and there are many intermediate shades and mixtures of 

 these colours. Some of the slaty varieties are of a purplish hue, 

 and this is occasionally spotted with green. Of the specimens I 

 collected, those of a coarse grain and of a dark reddish brown 

 colour, do not effervesce with acids ; those of a pale reddish brown 

 colour, and of a greenish grey colour, all effervesce, and some of 

 them briskly. None of the varieties of slate shew any signs of 

 effervescence. The magnet was not affected by any one of the 

 series. I did not discover a trace of any organic body in either 



* Similar alternations of clay slate with grauwacke liave been observed at Crinan 



and at Aberfoyle, by Dr. Mac Culloch. (Trajisactions of the Geological Society, Vol. 



II. p. 441, 447.) and by Mr. Jameson in the Pentland Hills. (Memoixs of the Werne^ 

 rian Natural History Society, Vol. II. Part I. p. J 81.) 



