OYSTERCATCHEB AND NATURAL ENVIRONMENT. 283 



the sea, and the breadth of the littoral zone is small. The 

 abruptness of the gradient is due to the passage of the deep north 

 channel (the old river-bed) close by the northern shore. The 

 exposure eastwards of Kinghorn to easterly and south-easterly 

 storms is severe, and the beach is mostly rocky and sea-worn. 

 The southern shore, on the other hand, descends gently into the 

 bed of a shallow sea, and the depth of the littoral zone is, in 

 most places, considerable. The greater part of the coastline is 

 sheltered from storms. To the east of Weak Law, however, the 

 shore falls more rapidly towards the sea ; the littoral zone 

 becomes narrower, and the rocks more and more sea-worn, as 

 the exposure to the effect of storms and the depth of the adjacent 

 waters increase. The distribution of the Oystercatchers is 

 evidently affected by the general features of the coastline, for 

 they are relatively scarce along the northern shore, plentiful on 

 the south coast westward of Weak Law, and less numerous 

 eastward on the open coast. 



Geologically, the south coast shows an ascending series of 

 rocks from east to west. Beginning in the Old Ked Sandstone 

 period, a change occurs in the vicinity of Weak Law to the 

 Carboniferous series, which persists to and beyond the western 

 end of the area. The sedimentary rocks have little importance 

 in the present connection. The abundance of durable volcanic 

 lavas and intrusions has a considerable indirect effect on the 

 Oystercatcher population ; the non-columnar form of analcite 

 basalt being especially favourable to the formation of Mytilus 

 settlements. 



The average temperature for the whole year at Leith (5*5 

 metres above M. S. L.) is 8*83° + 3*72° C. The monthly means 

 are 6-16° ± 3-00° in November, 4-11° + 2'83° in December, and 

 3-89° + 2-95° in January (1871-1905). 



The mean rainfall at Leith is 0*60 metre; the number of wet 

 days, 179 (1871-1905). 



The prevailing winds are westerly. 



The ordinary spring tide rises 3*7 metres at Dunbar, 

 4*6 metres at Prestonpans, 5 metres at Leith, and 5*8 metres 

 at the Forth Bridge. The increase in rise of tide from east to 

 west causes the foreshore to be deeper for equal gradients in the 

 higher reaches of the estuary. Full seas occur in late autumn 



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