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 innumerable 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
