December 2, 1889.] 



KNOWLEDGE. 



21 



AN ILLUSTRATED 



MAGAZINE OF SCIENCE 



SIMPLY WORDED— EXACTLY DESCRIBED 



LONDON: DECEMBER % 1889. 



CONTENTS. 



Rock Pinnacles. By Prof. G. S. Boflcier, F.L.S., F.G.S., &c. 

 Mall-Clad Animals. By R. Ltdekker, B.A. Cantab. 



The Common Flea. By W A. Bctler 



On the Scintillation of Stars By A. C. Rantard 



Notices of Books 



Growth and Decay of Mind. By tlio late R. A. Proctor ... 

 Periodical Comets due in 1890. liy W. T. Lynn, B.A., 



K.R.A.S 



R. A. Proctor Memorial Fund 



Letters:— C. A. Ychni;. W. H. Pickeuing. C. E. Peek, T. S. 



I'lrrrv, W. Westi.i.iutii 



The Strike, in its Relation to Health and Life. By 



Alex. B. MArDiiw,u.L, M.A 



The Face of the Sky for December. By Herbert 



Sadler, K.R.A.S. 



Whist Column. By W. Montagu Gattie 



Chess Column. Bv I. Gunsberg 



ROCK-PINNACLES. 



By Prof. G. S. BouLGER, F.L.S., F.G.S., &c. 



Fl'jW coni?iderations are more conducive to a due 

 sense of hitman littleness and of the grandeur of 

 Nature than a study of the manifold ways in 

 which physical causes will sometimes bring about 

 the same result. Towering over an English heath 

 or moor, or overhanging the sides of a romantic river- 

 worn ravuie, rising among Swiss pine-clad slopes, with- 

 standing the fury of Atlantic waves off the coasts of Orlmey 

 or Cornwall, or standing up out of the arid plains of 

 Colorado, we see phuiacles of stone, forming an integral 

 part of the neighbouring rocks, but hewn in divers ways by 

 Nature's hands. Often grotescjue in outline, suggesting a 

 squatting toad, a huge heathen idol, a Christian pulpit, or 

 the human form, they present to our inuigination — 



Shapes, 

 The sport of natui'o, aided by blind chance, 

 Rudely to mock the toiling works of man. 



This " blind chance " is, however, as in so many other 

 supposed cases of its operation, susceptible of .scientific 

 e.vamination. Varied as have been the forces at work, 

 and still more varied as their combined results may 

 appear, they will yet reveal their secret to the geologist, 

 obedient to the charm which he received from his master- 

 magician Lyell : " causes now in action are the same as 

 tliose in operation in the past." We propose to describe 

 some typical examples of these rock-pinnacles, and to 

 attempt the elucidation of their origin. First, however, 

 we nmst explain that by a " rock-pinnacle " we understand 

 a mass of rock, more or less tower-like, detaclied above, 

 but united below to other rock of the same nature. By 



this definition the consideration of wholly detached, or 

 stranded, masses is for the present put aside. 



Where our coast-lme is made up of soft or loose mate- 

 rial, as is mainly the case from Tees to Thames, we have 

 wide-sweeping bays, such as Bridlington, low cliffs such 

 as those of Suffolk or Sheppey and a retreating and 

 changing outline ; but few projecting pinnacles or rock- 

 stacks. The waves sweep everything away before them. 

 Where, however, we get even moderately hard rocks at the 

 sea-margin we at once have very different shores. Here 

 the coast rises in abrupt cliffs descending into deep water, 

 or a pebbly shingle extends at their feet : there foam- 

 tossing breakers indicate the submerged skerry ; and there 

 again rises perchance a solitary pillar marking the former 

 line of shore, or with a neighbouring natural arch indi- 

 cating its origin. 



At the western extremity of the Isle of Wight are the 

 well-known Needles and the former continuity of the chalk 

 downs of the Island with those of Dorsetshire — ere the 

 sea broke its way into the valley of the Frome, converting 

 it into what we now know as the Solent — is marked by 

 the similar pinnacles, Old Harry and his Wife, the Spiral 

 Eock and the Barns, off' Bollard Head on the opposite 

 coast. The chalk of which the Needles are composed has 

 been tilted, so that its once horizontal beds are now nearly 

 vertical, and of the "joints " or di\-isional planes by which 

 it is traversed in addition to its liedding-planes — due, also, 

 as many geologists believe, to the strain to which the rock 

 has been subjected — the dominant ones, the " master- 

 joints," though at right angles to the bedding, rise also 

 almost vertically from the water. These two sets of 

 divisional planes have determined the direction of Nature's 

 quarrying, as surely as they determine that of man's work 

 in every freestone pit or in every coal-mine. But what 

 has been the quarrying agency ? At iirst we might be 

 inclined imhesitatingly to attribute it to the sea. 



The precipitous Old Red Sandstone cliffs of Caithness 

 and the Orkney and Shetland Islands, graphically described 

 by Dr. Archibald Oeilde, furnish apparently still more 

 striking instances of marine action. Here in the Race of 

 Pentland we have the fuU fury of the Atlantic. " On tlio 

 calmest day some motion of air always keeps playing about 

 the giddy crest of the precipices, and a surge with cream- 

 ing lines of white foam meets at their base. But when a 

 westerly gale sets in, the scene is said to be wholly inde- 

 scribable. The cliffs are then enveloped in driving spray 

 torn from the solid sheets of water which rush up the 

 walls of rock for a hundred feet or more. " The mere 

 weight of water in these waves is amply suthcient to detach 

 fragments of rock, amounting as it often does to three tons 

 on the square foot ; but the action is enormously intensi- 

 fied by the existence of crc\ices or lissures ui the rock. 

 Every such cleft or chink becomes, as it were, a hydraulic 

 press, in which the in-rushing wave acts on every side with 

 a force equal to that with which it strikes the face of the 

 rock. Perhaps even more effective, however, than this 

 direct pressiire of the water is the alternate compression 

 and expansion of air whicli it produces. It \yas such 

 suction of a retiring wave that, during a storm in 1810, 

 burst a door of the Eddystone Lighthouse outwards ; and 

 it is, no doubt, such suction that starts large blocks of 

 well-built masonry from their places, and will rapidly 

 enlarge a cavity so conmtenced. It is this action probably 

 that nutinly explains the quarrying of blocks over 18 tons 

 in weight out of the solid rock at 70 feet above sea-level 

 on the" liound Skerry, in the Shetlands, and the hollowing 

 out of long tunnels or " blow-holes. " even in granite coast- 

 lines, as in the Bullers. or boilers, of Buchan. Dr. deikie 

 and other competent judges tell us. however, that the 



