382 Dr. Boase's Inquiry into the Nature 



cal affinity, and of the attraction of aggregation, very clearly 

 point out the difference between these powers; and they must 

 not, as in the case of geological terms, be treated as " mere 

 affectation" ; for definitions are, as aptly styled by Greenough, 

 the standard weights and measures of scientific intercourse. 



The Professor next enters on the most important part of his 

 paper, concerning the " transverse cleavage of slate rocks," 

 as deduced from his investigations in Cumberland and Wales. 

 This cleavage he also considers to have been produced by 

 chemical action; and, that it is the only true cleavage, making 

 with the planes of stratification an average angle of from 30° 

 to 40°, being in no instance parallel to the true beds, though 

 sometimes making an angle therewith as low as 10°, and even 

 as 6°. De la Beche has stated that cleavage planes are some- 

 times at right angles to the strata; but the Professor is of 

 opinion that he has confounded cleavage-planes with joints, 

 of which he treats under a separate head. He also thinks 

 that the Continental geologists have not duly appreciated the 

 transverse cleavage; and that some English writers do not 

 always distinguish between a jointed and slaty structure, but 

 seem to consider a laminated structure parallel to the strata 

 as one of the cases of a slaty structure. These, however, he 

 contends ought never to be confounded ; for " they have little 

 in common; can, in ninety-nine instances out of a hundred, be 

 distinguished even in hand-specimens; and ought to be desig- 

 nated by separate names." 



This supposed facility of drawing distinctions between the 

 two kinds of structure has induced the Professor to offer de- 

 finitions of them, notwithstanding his wonted dislike to such 

 labours. P Before I conclude this section," he observes at 

 p. 479, M I cannot help recommending, not a new nomencla- 

 ture, but a more systematic use of old terms than we are ac- 

 customed to. Bed is always applied as the English synonym 

 of stratum, and the terms thick-bedded, thin-bedded, thick- 

 flaggy, thin-flaggy, and laminated, are words in common use, 

 and express well enough different modifications of stratified 

 structure. The term foliated, again, expresses very well the 

 peculiar structure of mica-schist, and the fine glossy undula- 

 ting layers of greywacke. But it would be well to describe 

 no structure as slaty or fissile except cases of transverse 

 cleavage, using the term slate for a perfect oblique cleavage, 

 and some such term as flagstone-slate for imperfect cleavage ; 

 and in like manner slaty-flagstone may describe a very thin 

 or laminated structure, parallel to the stratification. In this 

 way, foliated as distinct from laminated, and slaty as distinct 

 from flaggy, become terms of a definite meaning." 



