3 ° 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
materials in water, but to the deposition of coarser and finer detritus by suc- 
feature oft^-t < ?. f ? 1 ^ uent sli PP in 8 or ^g, is a characteristic 
feature of the detritus which has filled up ancient volcanic funnels A later 
explosion from some adjacent part of the same vent has given rise to the 
discharge of a coarse agglomerate (6), which with blocks sometimes six feet 
long, overspreads the earlier material. A third detrital accumula on fn the 
™ix e hasr SiS f g ^ much calcite in its 
Previn T 7 ° WU 7 a sli P which cuts across both of the 
previous deposits. A broad dyke (d) of vesicular diabase (augite-porphyry) 
traverses the vent, and is probably later than any of the other Jks i7the 
wit/ T l l T 1U r de tMS aCC0UUt ° f the ManX Carb °niferous volcanic rocks 
with a brief reference to the intrusive masses which form a prominent 
the broad fivk^h w ^ ^ P icturesc i ue headland of Scarlet Point 
distone ? , Q fo ™ s that Promontory may be traced for some 
w r t w^T d l 7 f 161 ; pamUel d7kes run in the same direction 
winch it will be observed, is also that of the chain of vents. It might be 
said that the vents are, as it were, strung together by a line of°dykes 
?tv r 1 P 7T SeS t f VerSe b0th the a gg lome ™^ and the bedded luffs' 
history ° tS h th ng ’ °? f T t0 a com P ar lively late part of the volcanic 
histoiy. That they are truly intrusive and not lava-flows is, I think 
clearly shown by their vertical walls which descend through the surrounding 
rocks, and by the greater closeness of their texture, as well as the dirninm 
admitted ^hat 6,0 tV* 1 * VeSldeS f ong the contact surfaces. But it must be 
admitted that in their remarkably developed vesicular structure they look 
moie like streams of lava than ordinary dykes. 
It is this structure .which gives to these dykes their peculiar interest 
Bands of vesicles, from an inch or less to several inches in breadth run 
along the dykes parallel to the outer walls. Unlike the familiar rows of 
httle amygdaloidal cells m ordinary basalt dykes, such as those of the 
tury senes in Scotland, these vesicles, though small and pea-like in the 
narrower bands towards the margins of the dykes, became so large numerous 
iT;^ aderand ~ ai b " --7= 
While the intrusive material has for the most part risen in the form of 
dykes, in one part of the coast-sec- 
tion, a little to the west of Scarlet 
Point, it has been injected as a sill 
among the bedded tuffs. 1 A section 
taken at this locality gives the 
structure represented in Fig. 189. 
Fio. 189. Section of dyke and sill in the tuffs west 7' ^ f 3 ® ° f ' the 8 Teafc ^yke, 
of Scarlet Point, Isle of Man. ttle stra ta 01 tuff wdlich dip under 
, , p , , . , lt > r °P over and support an outlvino' 
sheet o the same material. The slaggy structure of parts of this sill give 
1 It is this sheet which has been described as a lava-stream. 
