The aim of geology is to describe accurately the long succession of changes in the crust of “this cooling cinder,” the earth, and to assign them in an orderly way to their causes. Hence, it calls upon nearly all other branches of science for help--astronomy, physics, chemistry, mineralogy, botany, and zoology. At the same time, it is essentially a recreative pursuit, for, as Mr. Horace Woodward says in his _History of the Geological Society of London_--published by the society--“the fulness of the science can never be attained without the vivifying influence of mountain and moor, of valley and sea coast.” It is owing to this that the soldiers of the hammer, from Murchison, Sedgwick, Lyell, Ramsay, Etheridge, Salter, onwards to the present generation of “stone-crackers,” are amongst the happiest, most genial, and mentally alert of our men of science. That word “stone-cracker” I take from a letter addressed to me when I was a boy of twelve by the Rev. J. S. Henslow, Professor of Mineralogy and later of Botany at Cambridge, founder, with Adam Sedgwick, the great Woodwardian Professor of Geology, of the now flourishing Cambridge Philosophical Society, and the teacher, guide, and fateful friend of Charles Darwin. It was he who sent Darwin on the voyage of the _Beagle_. I had met this wonderful old naturalist at Felixstowe when exploring the marshes for rare plants and insects with my father. My father was a first-rate man at a country walk, and could tell you all the time about the flowers, flies, stones, and bones you might encounter. But Henslow surpassed him. I remember to this day nearly every word Henslow said, and everything he did on that memorable afternoon nearly fifty years ago. Amongst other things he explained how the rough flint implements recently discovered in river gravels--proving man’s great antiquity--could be shown to owe their shape to blows, each blow causing a “conchoidal” fracture. And he struck with his hammer some very large flints which were lying in a heap in the meadow, and produced the most perfect dome-like broken surface or bulb of percussion. He promised to give me a real palæolithic flint implement and also a geological hammer. The letter which reached me later in London ran as follows: “Dear incipient Stonecracker--Enclosed you will find a draft for 10_s._ with which, at the shop in Newgate-street, you can obtain a geological hammer identical in all respects with my own.... In a separate parcel I send you a flint implement which I obtained myself in the gravel pit at St. Acheuil....” The hammer, the flint-axe, and the letter are to this day treasured with deep affection and reverence for the giver, by the boy who was thus so kindly initiated in the “art and mystery” of Stone-crackers. Henslow died in 1861 at the age of 65. His daughter was the first wife of Sir Joseph Hooker, the great botanist and traveller, who celebrated his ninetieth birthday in July, 1907, and is still in