Vol. 69.] VOLCANIC ROCKS OJ? THE FORFAESHIRE COAST. 475 



The minerals that fill the amygdaloidal cavities have beer 

 described by Heddle. 1 The commonest are calcite, chalcedony, 

 and chloritic minerals (celadonite and saponite). 



(4) Structures. — The great majority of the rocks consist of 

 pheuocrysts set in a ground-mass of felspar-laths — generally with 

 some augite — and interstitial glass, which is sometimes more 

 abundant than the minute crystals of the ground-mass. * The 

 ground-mass is holocrystalline in some of the rocks. 



Well-marked flow-structure is usually indicated by the arrange- 

 ment of the felspar-laths and by the elongation of the amygdaloidal 

 cavities. 



Microporphyritic structure is frequently seen, and one rock-type 

 is characterized by an abundance of glomeroporphjritic aggregates. 

 Glomeroporphyritic structure is occasionally met with throughout. 



Some very fine-grained rocks occur, in which the pheuocrysts are 

 similar in size to the minerals in the ground-mass of the usual 

 type. One example is extensively developed inland at Compass 

 Hill, Eriockheim, and Wuddy Law. 



(b) Dykes. 



The only evidence of intrusion among the rocks in this area 

 is furnished by a series of dykes of Lower Old Eed Sandstone age 

 north of Lunan Bay. All these rocks are much altered, but 

 appear to be very similar. They may be called porphyrites. 

 The one that is least altered occurs near Pishtown of Usan. Its 

 felspar-phenocrysts are probably andesine, and are full of glass- 

 inclusions and iron-oxides. Some doubtful pseudomorphs which 

 occur may have been pyroxene, but now consist of silica. The 

 ground-mass includes some stout plagioclase-erystals as well as 

 felspar-laths, magnetite, haematite, and dusty interstitial glass. 

 Some small patches consist of haematite enclosing the felspars 

 poikilitically. The numerous amygdales contain chalcedony, 

 quaitz, and chlorite. 



The other rocks are finer in texture, but possess similar minerals 

 and structures. In addition, they contain a yellowish- white 

 opaque mineral which is apparently leucoxene after ilmenite. The 

 supposed pyroxenes are few and small in proportion to the felspar, 

 and there is no suggestion of olivine having been present, so that 

 the?e rocks are much less basic than the lavas. 



IV. The Sediments Associated with the Lavas. 

 (a) Petrography of the Sediments. 



The sedimentary material varies in colour from pale green, 

 through various shades of grey and brown, to bright red. It is 

 fine-grained, compact, and usually hard, resisting the action of the 



1 ' Mineralogy of Scotland ' vol. ii (1901) pp. 138-140, 145, &c. 



