PLATE XXII. 



FIG. 1. 

 AN ALCIME -DIABASE. 



CAB CRAIG, FIRTH OF FORTH. 



Magnified 25 diameters. 



The minerals represented are augite (7), plagioclase (10), magnetite (5) 

 brown mica (9), analcime (17) and viridtte (18). An undetermined zeolite (.r) is 

 also represented. 



The augite varies in tint from nearly colourless to violet-brown. The 

 coloured varieties are markedly pleochroic in violet-brown and yellowish brown 

 tints. The plagioclase is much decomposed, and rarely shows well-marked 

 structure. Aualcime is very abundant in the rock. The undetermined zeolite 

 gives straight extinction and possesses strong double refraction. The major axis 

 of depolarisation lies parallel with the length of the section. The brown mica 

 is abundant in the most altered portions of the rock. Its boundaries are often 

 vague and ill defined. The extraordinary abundance of analcime seems to imply 

 the existence of nepheliiie or a felspar rich in soda. This rock might also be 

 called a teschenite. 



FIG. 2. 

 MELAPHYBE. 



NEAR POOR HOUSE, E. OF KINGHORN, FIFESHIRE. 



Magnified 50 diameters. 



The substances represented are plagioclase (10), chlorite (12), pseudomorphs 

 after olivine (6), interstitial matter (18), and magnetite. 



The felspar gives lath-shaped sections and shows twin striation. The 

 chlorite occurs in extremely minute scales often grouped in vermicular forms. 

 The interstitial matter is crypto-crystalline. The serpeiitinous pseudomorphs 

 after olivine are traversed by strings of magnetite which represent the original 

 cracks from which serpentinisation proceeded. No augite is present in the rock. 

 Its place is taken by chloritic aggregates. The term diabase is used for the 

 altered holocrystallirie dolerites ; and melaphyre for the altered dolerites which 

 originally contained interstitial matter. 



