Vol. 52.] BASALT-PIATEAUX OF NORTH-WESTERN EUROPE. 



:S45 



as to bury it entirely, and to provide a level floor for the piling up 

 of the succeeding sheets of basalt. 



The second vent, which is represented in fig. 9, exhibits the 

 same features, but with some 



Fi< 



9. — Volcanic neck close to that 

 shown in PL XV. &Jlg. 8. 



additional points of interest. 

 It measures roughly about 20 

 yards in diameter at the sea- 

 level, rises through the same 

 group of banded basalts (a a), 

 and is filled with a similar 

 agglomerate (b). Its more 

 northerly wall is now coinci- 

 dent with a line of fault (h) 

 which ascends the cliff, and 

 probably marks some subsi- 

 dence after the eruptions had 

 ceased. The southern wall 

 shows that a dyke of basalt (g) 

 has risen between the agglome- 

 rate and the banded basalts, 

 and that a second dyke (g) tra- 

 verses the latter at a distance 

 of a few feet. In this instance, also, the upper surface of the 

 agglomerate forms a cup-shaped depression which has been filled in 

 by two successive streams of lava (c, d). Among the succeeding 

 livas (e) a prominent sill (/) has been intruded, to which more 

 special reference will be made in the sequel. 



These necks are obviously volcanic vents belonging to the time of 

 the basaltic eruptions. They have been drilled through the basalts of 

 the lower part of the cliff, but have been buried under those of the 

 central and higher parts. The arrangement of their component 

 materials in rude beds dipping towards the centre of the vent shows 

 that the ejected dust and stones must have fallen back into the orifice 

 so as to be rudely stratified towards the centre of the chimney, which 

 was finally closed by its own last discharges of coarse detritus. The 

 saucer-shaped upper limit of the agglomerate seems to indicate that 

 after the eruptions ceased each vent remained as a hollow or maar 

 on the surface of the lava-fields. And the manner in which they 

 are filled with successive sheets of basalt shows that in course of 

 time other eruptions from neighbouring orifices gave forth streams 

 of lava which, in flowing over the volcanic fields, eventually buried 

 and obliterated each of the vents. 



In the destruction of the precipice some of the vents have been 

 so much cut away that only a small part of the wall is left, with 

 a portion of the agglomerate adhering to it. The third neck, for 

 instance, affords the section represented in fig. 10, p. 346, where the 

 horizontal sheets of basalt (a) have still a kind of thick pellicle of 

 the volcanic detritus (6), adhering to what must have been part of 

 the side of the orifice of eruption. The waves have cut out a cave 

 at the base, so that one can, by boat, get behind the agglomerate 

 and see the margin of the volcanic funnel in the roof overhead. 



