120 DR HATCH ON THE LOWER CARBONIFEROUS 



mass in about equal proportions. On the other hand, the rock on which the Hopetoun 

 Monument stands has an almost flinty texture with few and small felspars. 



With respect to chemical composition, these rocks contain from 60 to 63 per cent, of 

 silica. The alkalies generally amount to about 10 per cent., but in some cases are 

 lower. The magnesia is always less than 1 per cent., while the lime varies from 2 to 3 

 per cent. The ratio between potash and soda is not the same for all the types. Thus 

 in the Peppercraig rock the soda is slightly in excess of the potash, while in a specimen 

 from Kae Heughs the reverse is the case. The chemical composition of the Garlton 

 trachytes is thus in accord with that of well-known trachytes from other areas.* 



Examined under the microscope, these sections show that the rocks are composed of 

 large and remarkably limpid felspar crystals, imbedded in a minutely crystalline ground- 

 mass. The latter consists chiefly of felspar, but also contains granules of augite and 

 particles of magnetite. None of the rocks contain any interstitial glassy matter ; for 

 under a high power the ground-mass is invariably found to be completely crystalline. 

 One of the most striking points about these rocks is the decided separation of the por- 

 phyritic felspar from the microlitic felspar of the ground-mass. In no case could any 

 gradation or passage between the two generations be made out. There is no doubt that 

 we have here an instance of the crystallization of one constituent of the same rock-magma 

 under widely separated conditions. The porphyritic felspars represent the intra-telluric 

 conditions, that is to say, they were developed while the imprisoned magma simmered 

 below, prior to its escape upwards. On eruption, the cooling was quick enough to pro- 

 duce a uniform and even-grained ground-mass, but not sufficiently rapid to permit of any 

 of the magma consolidating as glass. As further evidence in favour of this view, attention 

 is directed to the remarkable zones of fresh felspar deposited round fragments of intra- 

 telluric felspars broken on eruption. 



Nature of the Felspar. — With the exception of isolated occurrences, the great 

 majority of the rocks contain porphyritic crystals of sanidine, showing the rectangular 

 form, Carlsbad twinning, and clear glassy habit characteristic of that mineral. A 

 cleavage flake (parallel to the face M) gave an extinction angle of 6-7°, measured to the 

 edge P.M. In many of the sections examined the crystals are as clear and unaltered as 

 in the most modern trachytes. Since the rocks are of Carboniferous age, this shows a 

 most remarkable resistance to the disintegrating action of the weather. 



Inclusions of augite, magnetite, &c, are abundant. These are often of a globular 

 character, and are irregularly scattered through the crystals or arranged in marginal 

 zones. The large amount of included matter occurring in marginal zones suggests that 

 a considerable portion was taken up during the continued growth of the crystals. 



The lath-shaped microlites and granules that make up the ground-mass also consist 

 of a clear glassy felspar, apparently of the same nature as the porphyritic sanidine. 



Although in most of the rocks the whole of the porphyritic felspar is orthoclastic, in 



* The Puy de Dome trachyte has Si0 2 , 62-83 ; K 2 0, 8"88 ; Na 2 0, 5-03— one of the Ehon trachytes ; Si0 2 , 63-40 ; 

 K 2 0, 3-54 ; Na 2 0, 8-39 (Kalkowsky)— trachyte from Monte dell' Imperatore, Italy ; Si0 2) 61-05; K 2 0, 5-28 ; Na 2 0, 

 5-94— trachyte from Monte Vettia ; Si0 2) 6187 ; K 2 0, 6-51 ; Na^, 5-07.— (J. Roth.) 



