192 MINERALOGY OF THE PENTLAND HILLS. 



porphyry, and of the porphyry imbedded in the 

 sandstone, are proofs, not only of the cotem- 

 poraneous formation of these two rocks, but 

 also of the chemical nature of the sandstone. 

 This porphyry is often amygdaloidal, and then 

 it is named amygdaloidal clinkstone-porphyry 

 The amygdaloidal cavities are more or less com- 

 pletely filled with mineral substances of different 

 kinds : sometimes they are entirely filled with 

 green -earth, or the green-earth merely encrusts 

 the vesicular cavities, and the cavity is filled up 

 with agate. The agate is associated with ame- 

 thyst, and also with common calccdony, which 

 latter is sometimes stalactitic, and is so circum- 

 stanced, as to shew, that although stalactitic in 

 form, it is not so in formation, being rather a crys- 

 talline shoot, than a stalactitic deposition. Some- 

 times the vesicular cavities are filled with a mix- 

 ture of calcareous-spar, heavy-spar, and brown- 

 spar. The agates and calcedonies are of consider- 

 able magnitude, some specimens being several 

 inches square. Some varieties of the rock con- 

 tain diallage. The porphyritic clinkstone is some- 

 times tufaceous, as in Glencross, and at first sight 



* Some of these varieties are grooved very deeply. These 

 groovings are seen sometimes on the surface of masses, some- 

 times in their interior, and we observe in this latter case that 

 the grooves fit into each other. I have noticed the bottoms of 

 the beds jsomewhat conroluted. 



