LITHOLOGY. 93 



noticed, some being distinctly Bedded, others Massive 

 or in large masses broken in varying directions. Some 

 are Jointed, divided by joints into blocks, or Columnar, 

 when the blocks resemble columns ; others are Amor- 

 phous, where no bedding or other structure is presented. 

 Some rocks are termed Slaggy, these are like furnace- 

 slag, others Scoriaceous, which are cinder-like in appear- 

 ance. 



Fracture. The character of the broken surface or 

 " fracture" affords useful indication of the kind of the 

 rock or mineral under examination, when taken in con- 

 nection with other peculiarities. The usual forms of 

 fracture are : 



Conchoidal Shell-like, as that of Flint 

 Even Flat Felstone 



Uneven = Eough Serpentine 



Splintery = Jagged Graphite 1 



Earthy = Irregular Chalk 



Lustre. After the texture of a specimen has been 

 determined, and its fracture noted, the " lustre " of its 

 freshly-broken surface requires observation. This char- 

 acter distinguishes a rock, whether it possess a crystalline 

 'or any other texture ; for instance, it does not at all follow 

 that because a rock is crystalline, its lustre must be 

 vitreous, or that it should not be such when the texture 

 is compact or granular. There are many kinds and de- 

 grees of each kind of lustre, the most usual being : 

 Vitreous, like glass 

 Sub-vitreous 

 Kesinous, like resin 

 Pearly, like that of a pearl 

 Silky, like silk 



