22 



KNOWLEDGE 



[December 2, 1889. 



fusillade of shingle hurled repeatedly against the cliffs is 

 probably yet more destructive than cither the weight of 

 water or the compression of air. 



Surely, then, when we find cliffs actually overhanging, 

 as do those of the Brough of Birsa, m Orkney, or the 

 Needles themselves, it is a sign of the undermining action 

 of the ocean. But no ; probably in every such case it 

 will be foiuid on examination that the rock forming the 

 cliff has joints or other divisional planes inclined at a high 

 angle inland. -Jomts or other divisonal planes, pre-existent 

 in the rock, and only revealed by the denuding agent, 

 determine, in fact, much of the outline of many \ery 

 varied kinds of rock-pinnacles. Thus, in spite of such 

 waves as those in the Pentland Firth above-mentioned, 

 the Old Man of Hoy towers above the adjacent cliff 

 from which it has been separated, a column of yellow 

 and red sandstone more than 600 feet high, and almost 

 four-square in its sharpness of outline. It consists, as 

 described by Dr. Geikie, of almost horizontal beds of 

 Upper Old Red Sandstone, resting on a pedestal composed 

 of volcanic rock above and of inclined Caitluiess flags 

 below. 



So also the limestone stacks off' the south coast of Pem- 

 brokeshire withstand the force of the Atlantic, though the 

 lines of beddmg and jointmg with which they are marked 

 tell a plain story of their former connection with the shore, 

 a story confirmed by natural arches such as that known as 

 " London P>ridge," which are simply intermediary stages 

 between the buttress and the pinnacle. 



Overhanging cliff's, however, are rare ; perpendicular 

 ones, far from common. In the majority of cases we find 

 they recede upwards : their upper portion wastes more 

 rapidly than their base : subaerial action is, in fact, 

 more potent m its more silent operation than all the com- 

 bined terrific forces of the ocean. The pulverising action 

 of the Sim evaporating absorbed water, the wind, which 

 sometimes tears large stones off the face of a cliff, and, 

 above all, ram and frost are the agencies at work. Here, 

 again, joints are of the utmost importance. It is down the 

 joints that the rain mainly percolates ; and it is thus along 

 the joints that the winter's frosts flake off' layer after layer 

 from our cliff's, irrespective of the mineral composition of 

 the rock. 



Subaerial action being thus more powerful than marine, 

 we might expect to find rock-pinnacles resulting from its 

 agency elsewhere than along our sea-coasts ; and so, 

 indeed, we do. In many of the beautiful dales of Derby- 

 sliire, where rivers have excavated for themselves deep 

 channels in the Mountain Limestone, huge buttresses of 

 this rock project fi'om the sides of the ravines, and some 

 of these buttresses sometimes stand out detaclied fi-om the 

 side wall of rock as solitary pinnacles. Here the bedding 

 is approximately horizontal, but rain-waters containing 

 carbonic acid and frost have acted first along one set of 

 joints, thus producing the buttresses, and then along the 

 other set (there are commonly two) at right angles, so 

 converting a buttress into a detached pinnacle. A pre- 

 cisely similar origin explains the Devil's Chimney, the 

 detached pinnacle of oolitic limestone that stands in front 

 of the Cotteswold escarpment near Cheltenham ; or the 

 Pulpit Eock of chalk at Bonchurch ; though these latter 

 have perhaps to some extent been imdermined and have 

 sliiipod forward in consequence. 



Where the Elbe cuts its way through the Erzgebirge 

 into Saxony we have, in the Saxon Switzerland, somewhat 

 similar i-esults in sandstone. Iimumerable pinnacles rise 

 above the forests, showmg distinctly their horizontal 

 bedding, but cut out into l)locks by their joints as if built 

 up of gigantic masonry . 



Granite " weathers," as the geologist terms this decay- 

 ing process, in a similar manner. Though not stratified, 

 it is frequently traversed by joints in horizontal as well as 

 in vertical directions, along which the disintegration acts. 

 Hard and indestructil)le as it appears, a very slight study of 

 granitic districts shows us that it is far from Ijeing as inde- 

 pendent of the weather as it seems. Whilst microscopic 

 examination exhibits incipient alteration of the felspar of 

 much granite near the surface into china-clay or kaoUn, 

 one can go into many a pit on Dartmoor and find what 

 appear to be walls of granite rotted into a gravelly con- 

 sistency, quartz, mica, and felspar embedded in soft clay 

 resulting from the decay of the latter. Such decay is 

 generally explained by the action of carbonic acid in rain- 

 water. This converts, it is said, the silicate of potash into 

 a soluble carbonate and leaves beliind the silicate of 

 alummium in a hydrated condition — in fact, as clay. 

 There are, however, difficulties in the way of so simple an 

 explanation, and M. Daubree has maintained that some 

 fluoride, probably hydi-o-fluoric! acid (which might well be 

 supplied by the tourmaline, fluor, or lepidolite that are 

 not uncommon as accessory minerals in granite areas) has 

 been the chief agent in attacking the felspar. 



Be the agency what it may, granite weathers on the 

 summits of hills into remarkable piles of stones, the weU- 

 known "tors" of Dartmoor and of Cornwall, that rise 

 from the roimded hill-tops like irregular fortifications or 

 " Druidic " monuments. Hey Tor, Rippon Tor, and 

 Helmen Tor are familiar examples of such stone-heaps ; 

 whilst the Cheesewring, north of Liskeard (a granite pUe 

 about fifteen feet in height), is further mteresting from 

 lia\-ing been considerably imdercut, being thus far wider 

 across above than at its base. This decay along horizontal 

 joints, sometimes accompanied perhaps by the action of 

 blown sand, has so undermined some of these rock-masses 

 that they can be rocked with ease upon their natural 

 pivots. This is the case with several of the stones on 

 Helmen Tor and with other granite masses in Devon and 

 Cornwall, to which the generic name of Logans is applied. 

 The best known of these is that near Castle Treryn, St. 

 Leven, a mass weighing about 6.5 tons and over 30 feet m 

 circumference. It rocks only in one direction, and loose 

 gravel scattered over the pivot indicates the wear and tear 

 still in progress. 



The mention of this gravel brings us to the consideration 

 of one agency very important even when acting alone, ^^z. 

 wind-blown sand. Li our climate it is difficult to ehminate 

 rain and frost ; but there are cases in which we may per- 

 haps safely put down the greater part of the action to 

 sand. Naturally these are among sandstone rocks. Near 

 Pateley Bridge, Harrogate, are the Brimham Rocks. They 

 are eniiiiidscd of Millstone Grit, the various layers nf wliich 

 diffrr ((insiili rably in compactness. The h;ii(lir layns 

 project IIS Icilges; but those near the top of tlic d.^taclu'd 

 pinnacles project far beyond the base, aroimd which there 

 is a plentiful supply of loose grit. One of these rocks has 

 been cut by Nature into the stndilance of some huge mis- 

 shapen idol, and another is a)i]no|iriati'Iy named the Pulpit 

 Rock. More than one of them i-m lie rocked. To realise 

 the cuttmg efl'ect of the blown sand, it is only necessary 

 to hold one's face near the base of one of these exposed 

 monoliths during a lugh wmd. 



Quartz sand, propelled by a steam blast at a pressure of 

 300 lbs. per square inch, will, in 25 muiutes, bore a hole 

 li inches in diameter and 1^ inches deep in a solid block 

 of corimdum or sapphire, though this substance is 9 in 

 von Mobs' scale of hardness, whilst the quartz is but 7 ; 

 and though wind in a strong gale, travelling 70 miles per 

 horn-, exerts a pressure of only 16 lbs. per square foot, 



