1846.1 SHARPE ON SLATY CLEAVAtJ^^^^^'^'* 89 



says in the sentence following that already quoted, " The dip is as 

 various both in angle and direction (that is, sometimes being in- 

 clined to one side and sometimes to the directly opposite side) as 

 the strike is uniform*." And again at p. 164, speaking of gneiss, 

 mica slate, &c. : " As in the case of cleavage, the angle of the dip in 

 foliated rocks is generally high, but variable, and alternates from 

 one side of the line of strike to the other, being sometimes vertical." 

 Again, describing the mount at Monte Video, he states, *' It con- 

 sists of hornblendic slate, which has an east and west nearly vertical 

 cleavage :" " the laminae on the north and south sides near the sum- 

 mit dip inwards, as if this upper part had expanded or bulged out- 

 wards." Again in more general terms at p. 164 : " On the flanks 

 of the mountains, both in Tierra del Fuego and in other countries, 

 I have observed that the cleavage planes frequently dip at a high 

 angle inwards, and this was long ago observed by Von Buch to be 

 the case in Norway : this fact is perhaps analogous to the folded, 

 fan-like, or radiating structure in the metamorphic schists of the 

 Alpsf , in which the folia in the central crest are vertical, and on 

 the two flanks inclined inwards." Again at p. 135 : "I suspect that 

 the varying and opposite dips may be possibly accounted for by the 

 cleavage laminae, though to the eye appearing straight, being parts 

 of large abrupt curves, with their summits cut oflf and worn down." 



These are all the statements met with tending to elucidate the 

 subject. Following up this information by my own observations, I 

 find that not only is the strike of the planes of cleavage uniform for 

 great distances, but on the same line of strike the direction of their dip 

 is the same, and its angle of inclination nearly uniform ; so that each 

 plane appears to hold its course at a certain inclination with great 

 regularity. Lines many miles long will be found along which the 

 cleavage planes are vertical, and on each side of such a line of ver- 

 tical cleavage the planes usually dip towards that line, first at a very 

 high angle, then at an angle which gradually diminishes in inclina- 

 tion as the distance from the vertical line increases ; thus producing 

 the fan-like arrangement noticed by Mr. Darwin. This regularly 

 descending series of planes being found on each side of parallel 

 lines of vertical cleavage, the two series either meet in the centre in 

 a sort of anticlinal axis, or coalesce into an arch ; thus forming the 

 figure represented by the section, fig. 18. The irregularities which 

 disturb this arrangement are so trifling in comparison to the general 

 uniformity, that they may be disregarded in all theoretical reasoning. 



Thus the arrangement of the planes between two lines of vertical 

 cleavage appears to form a complete whole, and the area bounded 

 by two lines of vertical cleavage n)ay be considered as belonging to 



chain, and endeavours to account for it. The Appalachian chain appears analo- 

 gous, as regards position and cleavage, to the chain of Snowdon, which will 

 presently be described (p. 90). The explanation fails, being built upon a partial 

 view of the position of the cleavage planes. 



* Professor Sedgwick says, in the memoir already quoted, " the planes of clea- 

 vage are inclined at various angles to the planes of stratification." 

 ' t Studer, in Edinburgh New Philosophical Journal, vol. xxxiii. p. 144. 



