202 TRANSACTIONS, NATURAL niSTORY 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 S.S.E, direction, 

 and the fourthj in the south-west, runs N.W. and S.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 

 brincrini!: to the surface beds of different deojrees 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 heighi}, 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 colours, 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 relativelv 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 bv blown sand, which is ridsfed 



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



