494 OVERLYING ROCKS. 



without great negligence, with the schistose 

 structure of argillaceous slate. 



The colours of clay stone are various ; as, pale 

 greyish, or muddy white, or ochre yellow of 

 various intensity, or flesh colour, or purplish, or 

 various tints of grey, from smoke coloured to 

 dark lead-grey, or nearly black, 



This variety occurs in veins, but rarely : it is 

 more common in mountain masses. 



D. Indurated claystone. Harder than the for- 

 mer, and distinguished by the superior lustre and 

 acuteness of the fractures, which are also, granular, 

 splintery, or conchoidal. It is here separated, rather 

 on account of its geological importance, than of its 

 mineralogical distinctness. It is among the most com- 

 mon and extensive of the trap rocks in Scotland. 



a. Massive, irregular. 



b. Prismatic, or columnar. 



c. Laminar under the same circumstances 



as C. c. 



The very dark varieties, particularly when 

 found in veins, or in a prismatic form, are some- 

 times called by the name of basalt, and may be 

 considered as at present forming one of the 

 varieties generally ranked under this indefinite 

 and popular term. It occurs, like the former, 



