40 GEOLOGY OF THE LOTHIANS. 



ly a foliated structure and a glimmering lustre ; very ge- 

 nerally there is associated with clinkstone an approxima- 

 tion to a schistose arrangement. In this case, however, 

 the same appearances which indicate that the causes of the 

 tabular form are in no respect similar to those which pro- 

 duced the stratified, evince that the internal structure of 

 clinkstone differs completely from that of rocks which have 

 not resulted from a state of fusion ; it is the consequence of 

 a species of crystallization probably superinduced during 

 the consolidation of the rock. Clinkstone also occurs por- 

 phyritic. 



AUGITIC ROCKS, OR TRAP ROCKS. 



The Trap or Augitic Rocks, which form the next great 

 division of the unstratified or plutonic masses, are, it may 

 be generally stated, composed of the two simple minerals — 

 Augite and Felspar — all the varieties of the series being 

 produced by differences, either in the state of their crys- 

 tallization, or relative proportions. Though, in a minera- 

 logical point of view, the members of the trap series fre- 

 quently differ widely from each other, still the frequent tran- 

 sitions of the one into the other, are at once sufficiently 

 indicative that the formation of all has been the effect of 

 the same general causes, differences of structure or com- 

 position having been produced by slight diversities in at- 

 tending circumstances. As in the felspathic series, so in 

 the trap family, there are a few rocks which have the same 

 general characters, and which may be considered as rocks 

 which, by passing into each other, form all the varieties of 

 this family. Of all the compounds of Augite and Fel- 

 spar which occur in the Lothians, forming rock masses, 

 Greenstone appears to be the most generally distributed. 

 Its component minerals vary in size and colour, change 

 in colour, however, being produced generally by the aspect 



