Vol. 69.] VOLCANIC EOCKS OP THE FORFAKSHIKE COAST. 479* 



in a large cavity, 14 inches deep, in compact lava. A vertical 

 section of the cavity and its contents has fortunately been made by 

 wave-action at the foot of the cliff. The plane of the section is 

 such that the cavity appears to be completely enclosed by the lava, 

 without any obvious connexion with the surface. The lower part 

 only of the cavity contains sediment; the bedding of the sediment 

 is undisturbed, and the sediment is not altered where in contact 

 with the lava. It seems certain that the cavity was one* of the- 

 usual steam-cavities 1 in the lava which somehow came into com- 

 munication with the surface, and so became partly filled with fine- 

 sediment. 



The lowest part of the cavity contains well-stratified, compact 

 sediment without amygdales. Above this the sediment is full of 

 amygdales, consisting of calcite and chlorite, and the stratification 

 is broken up, portions of the finer-grained layers being set in the 

 solid mass at all angles. The upper part of the cavity is filled 

 with green chloritic material enclosing a large crystal of calcite.. 

 Obviously, the breaking up of the sediment was not due to external 

 pressure, because the walls of the cavity are intact and the 

 sediment at the bottom is quite undisturbed. The only explanation 

 that seems at all likely is that the sediment was deposited in. 

 water in the cavity, and before this was quite filled another sheet 

 of molten rock flowed over the surface of the already cooled lava, 

 in which the cavity occurs. The fresh access of heat boiled the 

 water in the cavity from the surface without being sufficient 

 to penetrate to its base, just as water may be boiled near the top of 

 a test-tube while the lower part is cool enough to be held in the- 

 hand. The steam was unable to escape, owing to the sealing- up 1 

 of the outlet of tho cavity by the lava that supplied the heat, and 

 the sediment consolidated with its upper layers full of vesiclea- 

 which, as well as the empty upper part of the cavity, ultimately 

 became filled with secondary minerals. 



Examples of small fissures filled with amygdaloidal sediment in 

 the base of a lava-stream have been observed near the ' Spectacle.'' 

 The fissures die out upwards, and it seems impossible for the- 

 sediment to have entered from above. I can only conclude that 

 the lava boiled the water in the sediment over which it was poured,, 

 and the wet sediment was forced up into the crevices in the nether 

 surface of the lava-stream. 



Amygdaloidal cavities have also been occasionally observed in 

 the lower portion of the sediment where it is in contact with the- 

 lava beneath. They occur frequently, too, in the sediment that 

 cements the loose blocks at the surfaces of lava-streams. It is 

 possible that some of these instances may be due to the escape- 

 of gases from the lava during the accumulation of the sediment. 



1 A similarly-shaped cavity, 1 foot in diameter, in the same lava only a. few- 

 feet away was lined with chalcedony, inside which was a layer of bipyramidal 

 quartz-crystals with a large hollow in the interior partly filled by crystals, 

 of calcite and seleriile. This is simply a rather large amygdale, occurring in a. 

 cavity which did not communicate with the surface. 



