HARDWICKE'S SCIENCE-GOSSIP. 



forests must cease, and the allotment be affected by 

 the 31st of December, 1810 ; that all forests must be 

 protected, and therefore fenced ; and no cattle, except 

 swine, be allowed to graze therein, nor any mowing 

 under the trees take place. That, as the cutting 

 down of forest ought not to be the chief speculation 

 in the purchase of landed property, no one who by 

 purchase becomes ownerof a forest, may during the 

 first ten years fell for sale in the same, unless the 

 Revenue Chamber, after inspection, has decided that 

 the felling will not be injurious. 



Having seen the wretched condition of the Danish 

 forest consequent on the prolonged mistreatment 

 which the enactment of 1805 brought to an end, it 

 has now to be shown how the trees, freed from 

 oppression of the cattle, enjoyed without hindrance 

 the bounty of nature and flourished with a vigour 

 before unknown. 



OBSERVATIONS ON CLEAVAGE. 



IN text-books on geology, cleavage is usually 

 represented as running in a number of straight 

 and parallel lines in masses of rock extending through 

 large districts. The beds are drawn folded in 

 different directions, but the cleavage-planes are 

 uniform in dip, and therefore cut the beds at all 

 angles. When viewed in a large scale the parallelism 

 of cleavage is remarkable, and, in small sections, is 

 rightly represented as a number of parallel straight 

 lines, but when we come to examine these planes 

 closely local variations are by no means infrequent. 

 Taking into consideration the diversity of the physical 

 and chemical composition of rock masses, it would be 

 indeed surprising if the cleavage-planes passed 

 through such masses in straight and undeviating 

 parallel lines. Such a condition is theoretically 

 possible in a perfectly homogeneous rock, and hence 

 the more homogeneous the nature of a rock mass the 

 fewer deviations there will be in the direction of the 

 cleavage-planes running through it. Rocks exhibit- 

 ing cleavage being composed of beds of very different 

 hardness, the planes will be bent when passing from 

 a hard bed to a soft bed or vice versA. But the 

 refracted planes preserve their parallelism, Now if 

 we find one set of cleavage-planes passing through 

 hard and soft beds, forming acute angles with another 

 set passing through the same beds a little distance off, 

 we may conclude there has been a local variation in 

 the direction of the force which has produced. the 

 cleavage. Let us assume, for the sake of simplicity, 

 that the force has been mechanical pressure only. 

 The result of this pressure, long continued, has in 

 many cases been (1) the folding of the beds into 

 anticlinal and synclinal curves, and (2) the production 

 of cleavage-planes perpendicular to the pressure forces. 

 We will now consider only the refraction of cleavage- 

 planes produced by the very varying degrees of hard- 



ness of rocks, leaving evidences of local variation in 

 the pressure-forces for another paper. 



We have an instructive example of refraction 

 figured in Science-Gossip for November 1S81 (figs. 

 144, 145, p. 245) ; here is another, taken from the 

 same district, Geol. Surv. Gt. Brit. 57 N.E. In the 

 S.E. corner of the sheet some yellow dots represent 

 Lower Llandovery rocks, consisting of " sandstones 

 and slates," or b*. Our example occurs in a beautiful 

 valley, on one side of which is situated the farm 

 marked Paut-y-Pedwen on the one-inch map. In 

 the quarter-sheet the beds are shown dipping E. 

 Those represented in fig. 30 dip 72 E., and consist of 

 alternate hard and soft bands. The cleavage-planes 

 of the two sets of beds actually dip at opposite points 

 of the compass. The bed 1 is nine inches thick, and 

 consists of hard silicious clay-slate. The cleavage- 

 planes in it dip 68° W., or form an angle of 40 with 

 the beds, and are of irregular character. The bed 2 

 is similar to 1, but somewhat thicker ; the cleavage- 

 planes running through it are parallel to I. The 

 layers or lamince, the result of the cleavage, are 

 roughly T | inch thick. The beds a and /3 are 

 very thin, consisting of a soft shaly rock ; the 

 cleavage is moreover uniform, dipping 81 E. or 

 forming an angle of9° with the beds. The lamince 

 are T ' a inch thick and less. Two, measured side by 

 side, were only ^ inch ; this gives a thickness of 

 only '03125 inch for each. The laminse are more- 

 over very straight, although, as might be ex- 

 pected, the planes occasionally run into each other. 

 The clay slate of which they are composed is dark- 

 coloured and very fine-grained. On the whole, these 

 laminae present a marked contrast to the coarse, 

 irregular, thick and light greyish ones of 1 and 2. 

 Through a strong lens the beds 1 appear to consist 

 of little irregular grains of quartz scattered through 

 darker-coloured clay-slate. I was able to detect 

 here and there minute specks of true pyrites, little 

 brown patches of oxide of iron (probably the result of 

 the decomposition of the pyrites), and thin plates of 

 talc. In the beds 1 and 2 the latter are often quite 

 large, and can be seen with the naked eye, they are 

 colourless or greenish. The beds a and j3 appear 

 to be of much the same composition, but the grain 

 being very fine, a higher power must be used for 

 their examination. There seems to be less quartz in 

 these beds. 



There is no evidence to show us whether the 

 cleavage-planes were propagated from E. to W., or 

 from W. to E., nor is this important. We may 

 assume, the strike of the beds and the anticlinal axes 

 being directed about N.N.E. (true), that enormous 

 pressure has been applied on the rock masses in a 

 W.N.W. and E.S.E. direction, and therefore that the 

 cleavage was produced simultaneously from N.N.E. 

 and S.S.W. The important fact remains that the 

 planes in passing from hard to soft, or from soft to 

 hard beds were bent or refracted as much as3i n . 



