196 TERRITORY AND REPRODUCTION 



they may fulfil the second and third condition, 

 will not answer the requirements of a breeding ■ 

 station. 



Of no less importance is the type of rock- 

 formation. Not every formation affords the 

 necessary ledges upon which the egg can be 

 deposited with safety — the face of the cliff may 

 be too smooth, or too jagged, or the shelves 

 may run at too acute an angle. Many of the 

 large assemblages of Guillemots in the British 

 Islands are found where the rock is quartzite, 

 mica-schist, limestone, or chalk. The reason of 

 this is that such rocks are weathered along the 

 planes of stratification, of jointing, of cleavage, 

 or of foliation^the strata being probably of 

 unequal durability — with the result that in- 

 numerable shelves, ledges, and caverns, which 

 are taken advantage of by the birds, form a 

 network over the face of the cliff. But only 

 those ledges can be made use of which are 

 placed at a considerable height above the water, 

 because, when the cliff faces the open sea, the 

 lower ones are liable to be washed in stormy 

 weather by the incoming swell and thus become 

 untenable. There is a small cove in the midst 

 of the most precipitous part of the breeding 

 station at Horn Head, wherein the shingly 

 shore shelves rapidly to the Atlantic and faces 

 to the west. Here, towards the end of July, 

 young Kittiwake Gulls can sometimes be found 

 washed up on the beach — some living, but in 

 every stage of exhaustion, others dead, and in 

 every stage of decomposition ; here is the young 



