54 THE TRAP FORMATION OF 



with the description given of that mineral. Its colour is greyish-black, 

 —its lustre is slightly glimmering, and it has a flat conchoidal fracture, 

 and is difficultly frangible. It is not here the rock of most common 

 occurrence, but I name it first, and marked it No. 1, because it is the 

 hardest. It does not rise above the surface, but occurs in beds where 

 the masses are of an uniform, egg-shaped figure, perhaps a foot and a 

 half in their longest diameter, or it occurs in beds, where the masses are 

 angular pieces, or cubes disfigured, not much exceeding a foot in mea- 

 surement any way, and closely set together without cement.— It seems 

 to be little liable to external decomposition, and its surface, which is 

 smooth and entire, is coloured a yellowish white. 



No. 2.— There is another basalt differing little from the last, except 

 that it has not the same tenacity, and its colour is soot black. It occurs 

 only in angular pieces. I mark it No. 2. 



No. 3. — Is another in colour like the last, but still softer, and which 

 splits, with a moderate blow, at natural joints, into small four-sided prisms, 

 coated with a blueish coating, like that often seen on newly wrought iron. 

 ■ — ^It is in the mass amorphous. 



No. 4. — ^Is a five-sided prism. When the bed of a rivulet or river is 

 composed of angular pieces of basalt or wacken set together in a pave- 

 ment-like form, the surfaces exposed to the double effects of intense heat 

 and moisture, will appear cracked into a variety of prismatic forms, and 

 occasionally it will appear such as No. 4. 



All these rocks seem to be, though not wholly, yet essentially com- 

 posed of an intimate mixture of felspar and hornblende in an earthy state ; 

 and the latter, or hornblende, is the mineral that characterises all the harder 



kinds, 



