1866.] 



JVKES OLD BED SANDSTONE AND DEVONIAN. 



359 



believe the Mortehoe, Simonsbath, and Exton slates to be above 

 those of the Lynton district ; I believe that it is next to impossible 

 to assign a thickness to the former with anything like the approxi- 

 mation to certainty we might possibly do to the latter, and that 

 their apparent may be vastly in excess of their real thickness. 



In making observations on the dip of the beds in a great section of slates, 

 even the most practised observers may be occasionally led astray. 



If the beds undulate vrhile the dip of the cleavage is steady, and if the sub- 

 stance of the slate be very homogeneous, it may easily happen that the stratifi- 

 cation is only vrell shown when it has a given relation to the cleavage, when for 

 instance they coincide, or when they are at right angles to each other, or when 

 they cross at some other angle, so that the particular mark of stratification which 

 the kind of slate possesses shall be least obscured by the cleavage. But, when, 

 in observing a long section, a number of these comparatively well-marked " dips" 

 occur at several intervals, all in the same direction, an observer is apt to sup- 

 pose that the intermediate parts dip also in the same direction, and base his 

 estimate of thickness on the width of the whole mass and its mean angle of in- 

 clination. It is possible he may be right, but it is also quite possible that he 

 may be wrong ; and that the section may only show the same comparatively 

 small group of beds undulating across it, as suggested in fig. 15, where the parts 

 showing the bedding distinctly are supposed to be those which coincide with 

 the cleavage. 



N. rift-. !''>. s. 



Fig. 16. 



a, b, c, d. Places where the stratification coincides with the cleavage, and is there 

 observable, while in the intermediate places it is not discernable, or vice versa. 



If in such a section the bedding were 

 well seen at a, b, c, and d, and not at all 

 observable in the intermediate parts, the 

 whole mass might be supposed to dip 

 south at 45°, and show a continuous suc- 

 cession of beds, instead of a repetition of 

 the same. 



While on the subject of cleavage I 

 may perhaps be allowed to mention a 

 structure I observed at Mortehoe, which 

 I took at first for stratification, but had 

 afterwards reason to doubt it; I have 

 often observed it before, and it is some- 

 times very deceptive, and may accord- 

 ingly lead to very erroneous conclusions. 

 Along certain parallel bands in the slate, 

 about 1 inch in width, and a foot or two 

 apart, there occurs a shake or crumple 

 in the cleavage-planes. Viewed in sec- 

 tion it would be as in fig. 16. 



It is, however, when observed on the 

 face of a bed that it is most deceptive, 

 as the change of tint resulting from the 

 difference in the angle of reflection of 

 the light gives to the little bands the ap- 

 pearance of a difference of colour in the 

 substance of the rock, so that they might 

 be taken for the true " stripe " of the slate. 



