CUMBRAY (LITTLE). MINERALS. 489 



two classes of rock is the same. As this substance, 

 under the several aspects in which it occurs in trap, has 

 not been described, I shall here take the opportunity of 

 enumerating all the varieties which have fallen under my 

 notice. 



When in the powdery form, it is seen varying from 

 a muddy green, by a gradual change, to pure white, while 

 it also occasionally passes to a bright yellow. When in 

 the scaly form, in which it resembles common chlorite, it 

 is sometimes found aggregated so as to fill the cavity, at 

 others investing the surface only, in a botryoidal manner, 

 or else projecting in laminar indefinite crystals into the 

 interior. In these cases the colour passes from the com- 

 mon tint of chlorite to a darker hue scarcely distinguish- 

 able from black. More rarely, it is of a purple brown, 

 as if it had undergone decomposition ; of which, never- 

 theless, it gives no other indication, presenting, on the 

 contrary, an unusual degree of hardness. In this case 

 also it is generally crystallized. It is sometimes found 

 without either the scaly or the pulverulent appearance ; 

 covering, like a varnish, the interior of a cavity, or the 

 exterior of a nodule of calcareous spar or of some zeolite ; 

 in which cases it is frequently of a bright verdigris green. 

 Lastly, it often occurs in stalactitic or arborescent forms ; 

 sometimes incrusted with a distinct deposit of chalcedony, 

 subsequently imbedded in a solid mass of the same mate- 

 rial, or else simply surrounded with the general matter 

 of the chalcedonic nodule in which it lies ; its progress 

 being always easily traced from the including rock. In 

 these cases the ramifications are often very minute and 

 intricate ; producing those well known ornamental sub- 

 stances often supposed to contain vegetable remains, and 

 popularly distinguished by the name of moss agates. 



