THE GEOLOGY OF PUFFIN ISLAND. 81 
the deposition of the beds.* The limestone always con- 
tains a certain percentage of insoluble matter such as 
silica and alumina; Strahan quotes an analysis of the 
rock from Flintshire which yields ‘915 per cent of such 
material; + that in the Puffin Island limestone I found to 
vary from ‘8 per cent.up to as much as 4°23 per cent in a 
very gritty band pointed out to me by Mr. Lomas. That 
these shale bands are due to the second cause is shown by 
their great irregularity and variation in thickness, by their 
soon thinning out, and by what is more important still, 
their occurrence on different horizons in the limestone 
within a few yards of a cliff face. A still more conclusive 
argument, however, is a thin band that may be seen 
running vertically through the limestone, and in the 
absence of shale overlying from which this crack could 
have been filled, this particular band at least must be due 
to the second cause. The limestone at the points where 
these bands occur, shows unmistakeable signs of slight 
subsidences produced by the removal of some of the rock, 
which have slickensided the joint planes, and probably 
also caused the two small faults seen in the N.E. section. 
One of these has a downthrow to the west of three feet; 
one further to the west can be traced inland by a series 
of rabbit burrows which it was suggested to me might 
indicate the presence of an igneous dyke; a careful search, 
however, along the hne pointed out has convinced me 
that no igneous rock occurs there, and in fact there is 
none such on the island, though a dyke occurs in the 
limestone beyond the life-boat house on the opposite shore. 
The purity of the limestone and the severity of the 
denuding agents to which the islands is exposed accounts 
for the absence of any soil over much of the island; there 
* Kendall, Geol. Mag., 1884, p. 552/3. 
+ Mem. Geol. Survey: } Sheet, 79 N.W., 1885, p. 10. 
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