chap. XXIX 
THE ISLE OF MAN 
27 
conspicuously volcanic in origin. With the exception of occasional blocks 
°f limestone, which range up to masses several feet, and occasionally se'vera 
yards, in diameter, the dust, lapilli and included stones consist entirely oi 
fragmentary basic lava, so persistent in its lithological features that we may 
re gard its slightly different varieties as merely marking different conditions 
of the same rock. The accumulation of pumiceous ash in this southern 
coast of the Isle of Man is one of the most remarkable in Britain. As Mr. 
Hobson has well shown, the matrix of this tuff consists of irregular lapilli, 
representing what may have been various conditions of solidification in one 
original volcanic magma. This magma he has described as an “augite- 
porphyrite ” or olivine-basalt. Some of the lapilli, as he noted, consist of a 
Pumice “ crowded with vesicles which occupy more space than the solid 
part ” ; others show nearly as many vesicles, but the glass is made I11 own 
% the number of its fine dust-like inclusions ; a third type presents t le 
ce Us and cell-walls in nearly equal proportions. The same observer ouiic 
that where the substance is most cellular the vesicles, fairly uniform m 
size, measure about a tenth of a millimetre in longest diameter. 
An interesting feature of the tuffs is the abundant occurrence of loose 
felspar crystals throughout the whole group up to the highest visible strata. 
These crystals, sometimes nearly an inch in length, appear conspicuously as 
n i.i, ~ Tiioxr arp. ko much clecaye 
white 
- spots on weathered surfaces of the rock. They are so much decayed, 
however, that it is difficult to extract them entire. On the most cursory 
inspection they are observed to enclose blebs of a greenish substance li ' 
the material that fills up the vesicles in the pumiceous fragments and m 
fhe pieces of cellular lava. , „ , „„„ 
I have not ascertained the original source of these scatterec e ®P a • 
In one of the dykes on the north side of the agglomerate at Scarlet I om , 
as was pointed out by Mr. Hobson, large crystals of plagioclase occur 111 ie 
nielaphyre, but the felspars in the tuffs and agglomerates differ so muc 1 
f 1 0111 these that we cannot suppose them to have come horn t ie exp osio 
of such a rock. I failed to detect any other mineral in detached crystals m 
th e tuffs, but a more diligent search might reveal such, and afford some 
grounds for speculating on the probable nature of the magma from the 
ex plosion of which the scattered crystals were derived. It is at least certain 
fhat this magma must have included a large proportion of plagioclase 
Cr ystals. 
Between the lapilli and the minute pumice-dust that constitute the 
matrix of this tuff much calcite may be detected. Though this mineia 
may have been partly derived from the decay of the felspar m ie ava 
fragments, I believe that it is mainly to be attributed to the intermingling 
°f fine calcareous ooze with the ash accumulated on the sea-floor, 
remarkable association of the same kind will be described m later pages 
fr°m King’s County in Ireland. That abundant calcareous organism 
Peopled the sea in which the Manx Carboniferous volcanoes were active is 
shown by the contemporaneously deposited limestones. The tuffs the - 
Selves are occasionally fossiliferous. Species of Sjpirifer, Productus 
