478 DR. A. J0WETT ON THE [Oct. 1913, 



over the moist sediment has been to buckle up, distort and break 

 the superficial layers, the lava sometimes forcing itself into the soft 

 sediment ; portions of the sediment were detached from the main 

 mass and carried along in the lower portion of the lava-stream, 

 assuming a roughly spheroidal form. Nodules of this kind 

 embedded in lava have been found, varying from an inch to over 

 a foot in diameter. 



The heating effect of the molten rock has been: — 



(i) To bake the sediment with which it came into contact,, 

 giving rise sometimes to new minerals, especially in the case where- 

 spheroidal lumps of sediment have been incorporated by the lava v 

 and consequently subjected to a higher temperature for a longer- 

 time. 



(ii) To boil the water in the sediment without allowing the- 

 steam to escape, thus producing spheroidal cavities within the 

 unconsolidated sediment. The whole of the sediment may be 

 rendered vesicular in this way, or the upper portion only. The 

 cavities appear to have originated in the coarser layers of sediment,. 

 where the spaces between the particles were necessarily larger and 

 where, in consequence, a greater amount of water would be stored.. 

 The finer layers are frequently bent and broken up in connexion 

 with the provision of more space for these steam-cavities. 



Some of the spheroidal masses of sediment included within the- 

 lava possess this vesicular structure, as well as the sediment below 

 the lava. The vesicular structure is also commoner in the 

 interbedded sheets of sediment than in the sediment filling up 

 the fissures, though it is not absent from the latter. 



The combination of mechanical movement with heating has 

 led to an elongation of the vesicles in the direction of flow, similar 

 to that of the gas-cavities in slaggy lavas. In one example, a mass of 

 sediment 3 feet in diameter and 9 inches thick may be seen to rest 

 upon a sheet of lava, the rock above and around it having been 

 completely removed. From its nether surface wedges of its sub- 

 stance project into crevices in the lava beneath. It is composed 

 of finely stratified, very fine-grained sediment, the outer layers of 

 which are roughly parallel to the external surfaces of the mass ;• 

 but in the interior the material is crumpled up, and the layer* 

 are separated one from the other and sometimes broken across. 



The narrowest cavities are filled with chalcedony; larger ones are 

 lined with chalcedony, with quartz in the interior ; and in the 

 largest, the order is chalcedony, then quartz, with calcite in the- 

 centre. In some cases the chalcedony is succeeded by radial 

 aggregates of needle-shaped green crystals with high double 

 refraction, probably epidote. 



It seems clear that, unless the cavities had been filled with gases 

 until the sediment solidified, they could not have continued to exist 

 under the pressure of the superincumbent lava. 



A detailed description of an instance of the effect of heat 

 unaccompanied by mechanical movement mav not be out of place 

 (PL XLV, fig. 4 & PI. XLVI, fig. 6). The sediment is contained! 



