HA R D WICKE'S SCIENCE- G OSS IP. 



99 



and dotting the face of the country like grey specks, 

 are fragments of ancient castles and abbeys, 



"Where Ruin dreary dwells: 

 Brooding o'er sightless skulls and crumbling bones. 

 Ghastful he sits, and eyes with stedfast glare 

 (Sad trophies of his power where ivy twines 

 Its fatal green around) the falling roof, 

 The time-shook arch, the column gray with moss, 

 The leaning wall, the sculptur'd stone defac'd. 

 Whose monumental flatt'ry, mix'd with dust, 

 Now hides the name it vainly meant to raise."* 



Eut the title of this paper reminds us that we have 

 no time to dwell longer on the many resources of 

 favoured Tenby ; we must hasten on to Giltar Point, 

 stretching out to the west of the town, while the tide 

 is on the ebb, or our morning's work will be spoilt, 

 and our homely dinner unearned. 



The long, low land that appears exactly opposite 

 is Caldy Island. The cliffs where we intend to linger 

 for an hour or so are formed of the mountain limestone, 

 which comes in contact with the old red sandstone 

 about a mile farther along the coast. In one part of 

 the projecting cliff, and near some beds that have 

 been quarried for building stone, there is a fissure 

 that the ordinary observer might even fail to notice, 

 but fraught with interest to the student of geology. 

 Let us examine this fissure, and see what facts we 

 can glean from it, and what inferences we can draw 

 from our observations. 



On one side of the fissure, or lining its " footwall," 

 there is a vein (see fig. 65), from 2^-4 inches thick, 

 that appears to be filled with massive calcite. But a 

 line is seen to run roughly down its centre, and a 

 closer examination shows that it is formed of large 

 rhombohedric crystals of that mineral very imperfectly 

 developed. A few feet below the surface of the cliff 

 a small vein branche'S off from the larger one, as 

 shown in the sketch. 



Others also branch off from it, and some small 

 white veins run through the " country rock," more or 

 less parallel with it, and a few are seen to run in a 

 cross-direction, occurring here and there in bundles. 

 The thickness of the whole fissure varies from 2-2i 

 feet. It widens out above and below. From A to 

 B it is filled with angular fragments of limestone 

 and red earthy matter. From B to a short dis- 

 tance below the level of the beach the contents have 

 been washed away by the sea ; in fact, the action of 

 the waves at high water has formed an underground 

 tunnel between this spot and the next cove. Lines 

 appear indistinctly running down the centre of the 

 vein or fissure, more or less parallel to the sides ; 

 hence the contents seem to have been arranged in 

 layers. The fissure runs or courses N.W. b. W. 

 or about 35° \V. of N., and it is visible on the 

 opposite or N.W. side of the cove. It dips north- 

 ward about 66^. The walls below B are in parts 

 extremely well defined, and are coated with recent 



* These lines were written by — may I venture to say ? — that 

 too-much abused poet, David Mallet. They occur in the first 

 canto of his ' Excursion,' parts of which were fully appreciated 

 by the poet Thomson. 



incrustations of calcite, in small and imperfectly- 

 developed crystals. 



Several feet to the N.W. the [fissure is only from 

 one foot to 14 inches wide. The walls here are 

 better defined, and the inclination is as high as 

 75 p. The vein of calcite is still present on the foot- 

 wall, and is zh inches thick. Miners have observed 

 over and over again that when a lode is nearly 

 vertical, or, in other words, when the "hade" is 

 very slight, it is more [likely to be narrow, the walls 

 are pretty sure to be better defined ; and, what is 

 of more importance still, the ore will be more solid 

 and more concentrated. This general law, then, has 

 been followed in the case of our fissure. 



If, leaving the fissure for a moment, we examine 

 the limestone beds, or surrounding " country rock, " 

 we shall find them to be of very variable thickness, 

 from a few inches to several yards. We shall 

 discover, moreover, that these beds course about at 

 right angles to the direction of the vein, or N.E. b. 

 E. (i.e. about 55° E. of N.) and that their dip is from 

 53.5-57.5° towards the S.E. (see plan, fig. 66). Now 

 the fissure is roughly parallel to the tidal lines left at 

 high and low water ; it runs across the jutting cliff, 

 which is formed of beds striking at right angles to 

 the tidal line. 



The origin of this fissure is now evident. It was 

 clearly produced after the solidification and upheaval 

 of the beds, and must have been formed compara- 

 tively very recently, when, probably, the projecting 

 portion of the cliff was continuous with that on the 

 N.W. side of the cove. 



By the slow undermining action of the sea, aided 

 by atmospheric influences, the portion C of the 

 cliff gradually broke away from D. At first a mere 

 crack or chink must have been produced in the 

 rock. This crack was enlarged by the action of 

 gravity until it became a vein two or three inches 

 thick. This vein was subsequently filled by calcite, 

 dissolved out of the limestone beds above. The 

 fissure was then enlarged, and gradually filled with 

 debris from above, as proved by its present structure, 

 and by the lines running more or less parallel with 

 the walls. This fissure cannot extend to any depth ; 

 it is superficial, and due only to sea-action. It has 

 been propagated and subsequently filled from above 

 downwards, and not from below upwards, and seems 

 to resemble those fissures that are termed "gash 

 veins " by practical miners. 



The veins on the outside of the hanging-wall of 

 the fissure will well repay examination. A number 

 of small ones are seen running more or less parallel 

 with the large fissure ; as many as six can be counted 

 in the space of one foot. A few veins, as has been 

 already mentioned, run across, almost at right angles 

 to these. Here is a case of part of an encrinite 

 stem having been twice split and thrown out of its 

 plane by minute fissures (fig. 67). Here, again, two 

 encrinite stems have helped to fill up a fissure ; they 



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