Tol. 69.] VOLCANIC ROCKS OF THE FORFARSHIRE COAST. 477 



No satisfactory evidence has been obtained, such as to prove that 

 :any of the sedimentary material among the lavas has accumulated as 

 the result of direct ejection from volcanoes. It might simply be the 

 product of the rearrangement in water of rock-waste derived from 

 an adjacent land-area, where gneissose or granitic rocks, or pre- 

 existing sandstones, and volcanic rocks of Lower Old lied Sandstone 

 ■ age, were exposed. 



The sandstones and fine conglomerates in the vicinity of fhe Red 

 Head, south of and above the highest lava in the scries, include a 

 ■considerable quantity of volcanic debris mixed with ordinary 

 isedimentary material. The volcanic fragments are very abundant 

 in certain layers, and absent from others. They are sometimes well, 

 rounded ; but angular fragments, probably andesitic or basaltic, are 

 always included. Quartz and pale and dark micas form the bulk 

 of the remaining material, but fragments of red sandstone and 

 chlorite-schist also occur. 



The cementing material consists chiefly of chalcedonic silica and 

 red iron-oxides. 



(b) Amygdaloidal Sediments. 



(1) The conditions of formation of cavities in the 

 sediments. — From what has been said as to the relations between 

 the lavas and the sediments, it may not be unreasonable to assume 

 that successive streams of molten rock were poured out into water 

 in which fine sediment was constantly accumulating. The per- 

 sistence of these conditions further suggests a gradual subsidence 

 of the area, which, however, was varied from time to time by 

 ■stationary conditions — as indicated by the contemporaneous decom- 

 position of some of the lavas; and by actual re-elevation and erosion, 

 of which the interbedded conglomerates and the irregular surfaces 

 upon which these conglomerates rest are unmistakable evidence. 



A flow of liquid rock over unconsolidated sediment containing 

 water can hardly take place without important effects being 

 produced upon both the lava and the sediment. The examples of 

 pillow-structure mentioned above indicate how the lavas have been 

 affected by being poured into water, for Tempest Anderson l has 

 shown that pillow-structure may be produced in shallow water. 

 Further, the same author 2 has described how the lava boils the 

 water into which it is poured, producing explosions which throw 

 masses of lava into the air. Such conditions will necessarily lead 

 to the accumulation of a layer of clinker-like fragments upon the 

 surface of the lava-stream. 



But, beyond examples of the baking of the sedimentary material 

 by molten lava, we have no instance in modern volcanic phenomena 

 yet described that furnishes a parallel to some of the peculiarities 

 -which may be observed in Forfarshire. 



The mechanical effect of the movement of the molten rock 



1 ' The Volcano of Matavanu in Savaii ' Q. J. G. S. vol. lxvi (1910) p. 632. 

 ■ 2 Op. cit. p. 639 & pi. li. 



