c hap. XXIX 
THE ISLE OF MAN 
3i 
Fig. 190. — Section on south side 
of vesicular sill west of Scarlet 
Point. 
it some resemblance to a true lava-flow. But it is the same structure 
which can be seen in the dykes, while the closer grain along the contact- 
surface further connects it with these intrusions. 
There is, however, a peculiarity about the development of the vesicular 
structure in this sill which I have not observed anywhere else. If we 
examine the southern side of the crag near its 
eastern end we observe that the successive bands 
°f vesicles are arranged in . the same direction 
as the surface of contact with the underlying 
tuffs ) precisely as they are ranged in dykes 
parallel to the bounding walls. So far the struc- 
ture is quite normal. But, moving a few yards 
Westwards, we find that the bands begin to curve, 
ail d, instead of following the contact surface, strike 
J t first obliquely and then at right angles, until 
We have the structure shown in Big. 191. The 
t’Etnds here vary from less than an inch to more 
than a foot in breadth, and where broadest assume 
a sla ggy texture. I sought in vain for any evidence of subsequent dis- 
turbance such as might have truncated these parallel rows of vesicles and 
Pushed the rock bodily over the tuffs. The perfect parallelism of the bands 
with the surface of the tuff at the east end, and 
the absence of all trace of a thrust-plane at the 
base of the sill, seem to show that, though the 
rows of vesicles were undoubtedly at first arranged 
parallel to the surfaces between which the intrusion 
took place, the mass, before completely consolidat- 
ing and coming to rest, was ruptured, and a 
portion of it was driven onwards at right angles 
to its previous line of movement. 
A consideration of the singularly slag-like 
structure of the injected masses in the tuffs and 
agglomerates leads to the conclusion that though 
what we now see of these rocks did not actually 
flow out at the sea-bottom in streams of lava, it 
was intruded so close to the surface that the 
^Prisoned vapours had opportunity to expand, as in superficial outflows. 1 
18 inference is in accord with that derived from an examination of the 
necks, wherein we find evidence of the probable survival of parts of the 
ac tual craters and volcanic cones. 
in .illustrative of the occurrence of the vesicular structure in superficial intrusions, I 
j n J . again cite the dyke which cuts the ash of the outer crater-wall of the Puy de Pariou 
wit > Uve . r SUe. The andesite of this dyke is in places as vesicular as the lava-stream 
p ar 1 "'filch it was doubtless connected, but the vesicles have been flattened and drawn out 
hay 1 I t0 tlle w& ll s °1 Ifi® dyke. In this instance it is quite certain that there could never 
q J 6 been a »y great depth of detrital material above the fissure into which the material of the 
was injected (see vol. i. p. 66). 
v// M 
191. Bands of vesicles in 
the same sill. 
