]74 DESCRIPTION OF THE STRATA. 



coated with a ferruginous crust ; and the interstices between them 

 are filled up with ferruginous clay, or decomposed basalt. 



The rock is not uniform in its qualities. Most of it exhibits, in 

 the recent fracture, a rough granular surface, of a dark blackish blue 

 colour, with a number of shining specks, sometimes of the same co- 

 lour, sometimes darker, and sometimes lighter. Very often the stone is 

 quite black, with scarcely any tinge of blue. The shining crystals are 

 frequently oblong and thin, like those in the blue limestone. We have 

 not perceived among them any olivine or augite. The rock is remark- 

 ably hard, and supplies excellent materials for making roads ; for which 

 purpose it is quariied in several places. It seems to owe a great part 

 of its hardness to the quantity of iron contained in it; from whence also 

 the blocks derive their rusty coating. Some varieties of the stone ap- 

 pear like a mixture of clay and iron, and abound so much with the lat- 

 ter, that their recent fractures often exhibit a grey iron colour, which 

 soon acquires a rusty hue, on being exposed to the weather. These 

 specimens are not so compact as the black or blue basalt ; but often 

 abound with veins or crevices ; many of which are filled, or lined, with 

 calc spar, while others are empty. Similar cavities sometimes occur in 

 the black basalt, containing crystals of calc spar; some of which are of 

 a dull white colour, while others are highly transparent and beautiful, 

 having laminated shining surfaces. Along with such crystals, we find 

 in some of the crevices or small veins, a soft substance, consisting of 

 very minute jet black grains, slightly adhering to each other, and having 

 a glimmering lustre. When a portion of this substance is subjected to a 

 red heat, the black colour is discharged, and it acquires a yellow brassy 

 or pyritous aspect, the grains having a metallic lustre, and adhering 

 more closely together. During this process there is no appearance of 

 ignition. The substance is evidently a metallic oxide, and is probably 

 the black oxide of iron, with a slight mixture of sulphur. Rhomboidal 

 crystals of glassy calc spar are imbedded in it. 



