202 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 
Since our return we have learned from the Geological Survey 
Map* that the rocks are of Lower Old Red Sandstone age, that 
the cornstone overlies the conglomerate, and that four faults 
traverse the island across its shorter axis, Two run nearly north 
and south, one towards the east has a N.N.W. and 8.8.E, direction, 
and the fourth, in the south-west, runs N.W. and 8.E, 
To these faults, the existence of one of which we had suspected, ' 
is partly due the differences in the aspect of the island, by 
bringing to the surface beds of different degrees of hardness. The 
high dip of the strata gives what is called a tumulated appearance, 
In the New Statistical Account of the Parish of Southend, to 
which Sanda belongs, it is stated that “The island of Sanda is 
composed chiefly of sandstone, and the sandstone of which it is 
composed is elevated to the north, the dip being to the southward, 
and varying from 15° to 20°. It thus forms hills of about 300 
feet in height, which, on some of the shores, are broken into 
cliffs of moderate elevation. One of these forms a very 
picturesque object, presenting a natural arch of considerable 
dimensions. The rock is reddish-grey, and is interstratified with 
slaty clay of various colou‘s, but chiefly of a grey hue, It has 
been largely used in the building of the Parish Church of Southend, 
and several principal houses in the country.” 
In this account the angle of the dip and the height of the island 
are both under-estimated. 
The natural arch | Pl. III.], near the lighthouse, is certainly most 
picturesque. Its origin probably dates back to a time when the 
land stood at a lower level relatively to the sea than it now does. 
The first breach may have been-made by the waves acting along the 
joints of the rock. Since the sea retired, the wasting has been 
continued by other denuding agents, and in the course of centuries 
large blocks have been broken off from the top of the arch and 
piled in wild confusion below. The arch is thus rising higher, 
and is likely eventually to disappear. 
Other evidence of the altered levels of sea and land is afforded 
by a raised beach, which does not look quite 25 feet above the 
present one. At the head of the bay on the north side of the 
island, this raised beach is covered by blown sand, which is ridged 

* The Geological Survey Report, which includes Sanda, is not yet published, 
