296 TERMINOLOGY. . 204. 



fluous to enter, in this respect, into a minute detail. With 

 regard to the dendritic delineations, however, it is worth 

 noticing that they are real imitative forms (. 183.), and 

 that therefore they do not refer to the mineral upon which 

 they are found : they may be only superficial, or be distri- 

 buted throughout the whole mass of the specimen. 



The delineations of the Florentine ruin marble, a com- 

 pound variety of rhombohedral Lime-haloide, represent on 

 a small SCA!C a very interesting phenomenon, which occurs 

 very often in nature on a larger scale ; this, however, is 

 a subject foreign to the Natural History of the Mineral 

 Kingdom. 



. 204. THE STKEAK. 



If we scratch a mineral with a sharp instrument, 

 either a powder will be produced, or the scratched 

 place assumes a higher degree of lustre. Both 

 these phenomena are comprehended -under the ge- 

 neral expression of the Streak. 



The lustre is increased by the streak in malleable me- 

 tals, in several species of the order Glance, and in several 

 varieties of black Mineral-resin. This is likewise the case 

 with clay, and with several other decomposed minerals. 



The best method for observing the colour of the powder, 

 is to rub the mineral upon a plate of porcelain biscuit, or 

 upon a file, till the powder appears. In those minerals 

 which are too hard for a process of this kind, the streak itself 

 is of no great consequence. 



Some minerals retain their colour in the streak ; others 

 change it. Among the former are most of those belonging 

 to the orders Glance, Haloide, Spar, and all those of a white 

 colour ; among the latter, several of the orders Ore, fy- 

 rites, Blende, &c. The former are said to be unchanged in 

 the streak ; of the latter, the alteration of the colour in 

 the streak is indicated. A white or grey streak of mine- 

 rals is said to be uncolourcti. 



