Ch. XIII.] THE INCLINED POSITIONS OF STRATA. 291 



same course, and selected the bearings of the laminae, because 

 it is a point easily determined, whenever a small portion of 

 rock is exposed ; and because there is an advantage in having 

 a certain criterion to guide us in forming our judgment. 

 But it does not follow that this selection is correct. It is true 

 that the position of the laminae frequently corresponds with 

 that which horizontal beds might assume, if tilted up ; but 

 this is not always the case, for the laminae sometimes dip 

 towards the granite, as well as in the opposite direction. 

 Again, it has been shown that these rocks are intersected by 

 other parallel planes, which cross the laminae at various 

 angles, but have the same bearing or strike ; so that, as the 

 planes of the laminae most frequently dip from the granite, 

 those of the other series incline most commonly towards this 

 rock. Now it has been already stated, that the fissile struc- 

 ture of the slates is perfectly independent of the so called 

 strata, since they are arranged at various angles thereto, in 

 rocks not only of the primary, but also of every other geologi- 

 cal epoch. Since, therefore, this structure does not infallibly 

 denote the supposed order of deposition, why determine on 

 the planes parallel thereto, in preference to those which have 

 an opposite dip ? There is one reason why the latter might 

 be preferred: viz., that these planes are for the most 

 part parallel with those of the massive rocks (greenstones, 

 porphyries, and the like) with which these slates are as- 

 sociated. 



In Cornwall, the lamellar system of planes seldom 

 attains great angles, but fluctuates between 20 and 4-0; 

 whereas the others vary from 4?5 to 70, and even higher 

 inclinations. 



What has been the rule for determining the selection in 

 other countries ? It does not appear that any fixed plan has 

 been followed : sometimes, however, it has been cursorily 

 remarked, that the laminae cross the planes of stratification. 

 On referring to Weaver's description of Ireland, in the fifth 

 chapter, it will be seen that the mica- schist and clay-slate, 



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