What is the result? At the bottom of the whole we place the granite of Stromness and Graemsay. This represents part of the floor of the old lake on which the gravel and sand and mud were laid down—a part which stood up above the water as an island. Next to this we find a thin layer of pudding-stone. This is formed of the old gravel which gathered on the beaches and shores around the granite island as it was slowly covered over. Above that were laid down the flagstones of the West Mainland; then those of Kirkwall, the East Mainland, and the North Isles; then the yellow and red sandstones of Eday, Shapinsay, the Head of Holland, Deerness, and South Ronaldsay. [Illustration: _Cliff showing horizontal strata._] The whole series of these rocks must be thousands of feet thick, and how long they took to form we cannot conceive. Then there is a gap in the series. This means that for a time the lake was dry land and instead of mud and sand being laid down, the rocks which had been formed were partly washed away by rain and streams. After a long time had passed, another lake was formed, and in it were laid down the yellow sandstones of Hoy, which are quite different from the other yellow sandstones of Orkney. When you think that each thin flagstone or layer of sandstone in our quarry was once a sheet of mud and sand, and that it took months, no doubt, or even years, to gather on the lake bottom, you can understand how vast a space of time is represented by the old red sandstone of Orkney. “Books in the Running Brooks.” Let us now take a stroll along one of the little burns which flow between their green or heathery banks in any of the valleys of our native islands. These little burns are very small in comparison with the mighty rivers of the world, yet they are quietly performing a great task, and in the long past ages the amount of work which they have done is far greater than you have ever imagined. It is summer, and the burn runs shallow and slow; the pebbles and sand show clearly in the pools. The burn enters a little bay, and as it flows across the shore it breaks up into several streamlets, each working its way through the gravel. Brackish water plants grow here; the shore is muddy, and the seaweed is often soiled with fine sediment. The burn has brought this down, and has dropped it where it enters the sea. We follow the channel upwards, through flat, rich meadows, which may be tilled, and covered with corn and other crops. In the meadow the burn winds to and fro, and in each loop the outer side is steep, often overhanging: under the grassy bank the trout lie hid. The inner side of the bend is shallow, slopes gradually down to the water, and is covered with small broken stones and gravelly pebbles. We can see that the current is eating away the steep outer bank by undermining it, while on the inner side the small stones are gathering.