XIV INTEODTJCTION. 



!N"ortliumberland and Durham form the north-east boundary 

 of England. They have a coast line more than a hundred miles 

 in length, from the River Tees at the south of Durham, to the 

 Tweed at the north of iN'orthumberland. On the Durham side 

 of the Tees is an extensive salt-marsh, with numerous narrow 

 ''stells" up which the tide flows. Much of the surface is 

 covered at high water of spring tides. Here grow the usual 

 salt-marsh plants, and here occur the few Lepidoptera peculiar 

 to such situations. From this point the coast is alternately 

 sand banks and cliffs, with a few large towns chiefly at the 

 mouths of the larger rivers. In some of the limestone cliffs 

 deep ravines have been formed by the action of water, which 

 have well-wooded sides, and afford a sheltered retreat for a 

 large number of insects. Castle Eden Dene is perhaps the 

 longest and widest of these ravines. It extends inland for 

 several miles, and has long been famous for the wealth of its 

 lepidopterous fauna. Westward from the sea the land gradually 

 rises, and after a wide area of cultivated land, well wooded in 

 some places, we reach moor and moss, still trending upwards 

 till we ascend the mountains on our extreme west, which 

 extend from the Cheviots in Northumberland to the high land 

 of Upper Weardale and Upper Teesdale in Durham. We have 

 thus a littoral fauna — the fauna more particularly pertaining to 

 cultivated land and woods — and the fauna of elevated moors 

 and of mountains. Yery few areas of like extent have so varied 

 a surface, nor greater ranges of uncultivated land, where nature 

 is little interfered with. JS'orthumberland runs, so to speak, 

 far into Scotland. The south of the county is as far north as 

 Dumfries in Scotland, while Tweedmouth, on the English side 

 of the Tweed, is further north than the Isle of Arran, which 

 produces so many curious forms of Lepidoptera ; yet none of the 

 species generally considered Scotch have as yet been taken with 

 us, nor is there any species among the larger Lepidoptera that 

 is peculiar to these counties. Polyommatus salmacis was sup- 

 posed to be confined to Durham. It was then thought to be a 

 distinct species, and was called the Durham Argus. It is now 

 known to be but a local form of a widely distributed butterfly. 



