285 



large portions of the volcanic products of Lower Old Red Sandstone and 

 early Carboniferous times. Those of the latter period have been described 

 by Dr. GEIKIE in his Memoir on the Carboniferous Hocks of the Firth of 

 Forth Basin. (1) They have been mapped as " felstones," " porphyrites " 

 and " claystones." They form the thick terraced masses which range 

 through the north of Ayrshire, Renfrewshire and Dumbartonshire to 

 the Forth at Stirling. Similar rocks in Berwickshire, Roxburgh and 

 Dumfries spread over wide areas at the base of the Calciferous Sandstone 

 series. The ground-mass is usually some shade of red or brown, varying 

 from a dark chocolate or purple tint to a pale yellow or nearly white ; 

 greenish and bluish shades being less common. In this ground-mass 

 crystals of felspar porphyritic more or less decomposed are very common. 



Amygdaloidal varieties of the rock may frequently be observed. The 

 specific gravity ranges from 2'6 to 27. Under the microscope the more or 

 less decomposed felspars are seen to have contained the inclusions 

 characteristic of the andesites. The ground-mass is an aggregate of 

 minute felspar microlites and grains and crystals of magnetite. Pyroxene 

 is sometimes recognizable as granules and more rarely as crystals. It is 

 very subordinate to the felspar and as a rule has been replaced by 

 secondary products. Ferric oxide is usually present in greater or less 

 quantity. Hence the prevailing reddish tints. Good illustrations of this 

 type of rock occur in the Garlton Hills of Haddingtonshire. 



Mr. DURHAM and Prof. JUDD have described some very interesting and 

 comparatively unaltered rocks occurring in the north-eastern portion of the 

 county of Fife, near the Tay Bridged They belong in all probability to the 

 Lower Old Red Sandstone period. The rock of Northfield quarry is dark 

 in colour (resembling a basalt) and without porphyritic crystals. The main 

 mass of the rock is composed of felspar microlites, pyroxene-granules and 

 a glassy base containing globulites, trichites and belonites. In this ground 

 mass lie groups of pyroxene crystals the angles of which are mostly 

 rounded. Two varieties of pyroxene were recognized ; monoclinic (augite) 

 and rhombic (enstatite). The specific gravity of the rock Avas found 

 to be 2-68. 



The rock of Causeway Head is more distinctly crystalline than the 

 above. Under the microscope it is seen to consist essentially of large 

 microlites of felspar, a rhombic pyroxene and some grains of magnetite. 

 The pyroxene sometimes occurs in fairly well-formed prismatic crystals and 

 sometimes as rounded or irregular grains. Some of the pyroxene may be 

 monoclinic. Biotite occurs as an accessory constituent. There are no 

 porphyritic constituents in this rock but the pyroxene grains are some- 

 times found grouped round the magnetite in such a way as to give the rock 

 a glomero-porphyritic structure. The rock has a specific gravity of 279 

 and is of a dark colour and therefore basaltic in habit. The specific gravity 

 and general appearance both indicate that we are dealing with a some- 

 what basic rock. Indeed the principal difference between this rock 



(1) Trans. Roy. Soo. Edin. 1880 p. 486. 



(2) Q.J.G.S., Vol. XLIII. (1880), p. 418. 



