132 Revietcs — J. H. L. Vogt — Rock-Magmas as Solutions. 



which increase in frequency and intensity towards the north-west. 

 All have exercised a marked control upon the river-system, a fact 

 which is well illustrated in the relation of the Tawe and Neath 

 Valleys to the two above-named disturbances. The sections across 

 the Vale of Neath disturbance (plate ii) present an extremely 

 efifective piece of stratigraphy. 



The glacial phenomena of the region constitute not the least 

 interesting portion of this memoir. In previous parts, descriptive 

 of the South Wales Coalfield, reference has been made to the 

 importance of the Brecknock Fans in this connection. The fact 

 that the Fans formed a centre of dispersion of boulders was first 

 ascertained about 20 years ago by Professor Edgeworth David, and 

 one result of the recent mapping has been to confirm his conclusions 

 in all important particulars. It is shown that the ice generated ou 

 the Old Red Sandstone plateau moved in a general southward 

 direction, surmounting the scarps of the Carboniferous Limestone 

 and Millstone Grit, and crossing deep valleys with little or no 

 deflection. It failed, however, to overtop the great Pennant scarp 

 of Carn Mosyn, but split against it, one part moving down the 

 Cynon and Tafi" Valleys in a direction somewhat east of south, while 

 another part turned south-westwards down the valleys of the Neath 

 and Tawe. Some morainic ridges, lying at the foot of the loftier 

 precipices, appear to have been the last work of the retreating 

 ice-sheet ; several of these enclose tarns. The Brecknock drift, 

 near its source on the Old Red Sandstone moorland, is composed 

 wholly of the detritus of that rock. Being an unstratified 

 deposit, with boulders but little rounded, it is classed as a Boulder- 

 clay, but having been derived from a formation in which arenaceous 

 material predominates it is of a gravelly character. Southwards, 

 within the Carboniferous area. Carboniferous boulders enter largely 

 into its composition, the quartz-conglomerates and grits of the 

 Millstone Grit being especially ubiquitous. In the coalfield, 

 where shales predominate, the deposit assumes the character of 

 a typical Boulder-clay, and the red gravelly matrix gradually gives 

 place to a stiff blue clay. These features are well illustrated by 

 a map (fig. 12), showing the watersheds, distribution of the drifts, 

 and the glacial striae. The Brecknock drift and the local drift are 

 distinguished by different symbols, and it is interesting to note how 

 both classes of drift have been deposited in large amounts along the 

 northern base of the Carn Mosyn massif, a portion hanging to the 

 Neath drainage on the south-west and a smaller portion to the Taff' 

 drainage on the south-east. 



III. — Rock-Magmas as Solutions. 

 Die Silikatschmelzlosungen. I. Uebke die Mineralbildung in 



SiLIKATSCHMELZLOSUNGEN. II. UeBER DIE ScHMELZPUNKT- 



ErNIEDRIGUNG DER SiLIKATSCHMELZLOSUNGEN. By J. H. L. 



VoGT. Vidensk.-Selsk. Skrifter, 1903, No. 8, and 1904, No. 1. 

 ri^HE analogy between igneous rock-magmas and saline solutions 

 J_ has been a subject of speculation since the time of Bunsen ; 



