SECONDARY ROCKS— MINERALOGICAL CHARACTERS. 25 



frequently of a black, or blackish-brown, containing oc- 

 casionally minute scales of mica. The colour of these two 

 slates is not uniformly sufficiently indicative of their distinct 

 natures, and as it is highly necessary,in an economical point of 

 view, that they should be easily recognised, we may remark, 

 that in the streak we have a character which at once de- 

 cides them. The slate-clay has a streak without lustre 

 and of a grey colour, while that of the bituminous shale 

 has a resinous lustre and a brown colour. The bitumi- 

 nous shale frequently makes a transition into coal ; exam- 

 ples of which are to be observed on the shore below Kirk- 

 aldy and Dysart. 



Clay-Ironstone, the next member of this series, is possessed 

 of several characters which render its examination a subject 

 of considerable interest. The clay-ironstone has in ge- 

 neral a blackish-grey colour, with a conchoidal fracture 

 and no lustre ; it occurs in two positions, either as strata of 

 inconsiderable thickness, alternating with the several mem- 

 bers of the coal formation, or in rows of spheroidal lenticular 

 masses, imbedded in shales, and arranged in lines parallel 

 to the direction of the strata. On fracturing the lenticu- 

 lar concretions of ironstone in a direction parallel to that 

 of the strata containing them, there is generally to be ob- 

 served an internal structure ; this structure consisting of a 

 series of veins of calcareous spar, diverging from a central 

 point with more or less regularity, which series is crossed 

 by another arranged round the centre of the clay-iron- 

 stone. The veins which produce this structure are ge- 

 nerally composed of calcareous spar, which is associated, 

 in some in stances, with quartz and elastic mineral pitch; they 

 all decrease in size from the centre to the circumference, 

 being at their origin often sufficiently wide to allow of the 

 crystallization of the spar in its usual forms. That these 



