of the Structure of Bocks. 383 



Now, let us examine into the practical working of this 

 " more systematic use of old terms." We will take, in the 

 first place, "hand-specimens" of the very fine felspar, acty- 

 nolite, hornblende and chlorite schists ; or of the Delabole 

 slate near Camel ford, which is perfectly fissile in the usual 

 sense of the word, affording roofing-slates second to none in 

 the universe. Are these slates'? Have they the slaty struc- 

 ture? Or, in other words, do they possess a perfect cleavage? 

 The Professor has said that this kind of structure ought never 

 to be confounded ; and yet it is impossible to answer this ques- 

 tion, according to the proposed system, without having pre- 

 viously ascertained the direction of the strata. It is there- 

 fore evident that the slaty structure cannot be distinguished 

 M in ninety-nine cases out of a hundred, even in hand-speci- 

 mens." Perhaps this example will be objected to as a Cornish 

 case — the one in a hundred — an exception to the general rule, 

 which may be discarded as unessential, according to another 

 canon of the Professor, which (at p. 481.) runs thus : " It is 

 the business of a geologist to consider both the resemblances, 

 and the differences of the things he describes ; and after a 

 broad view of nature's kingdoms, he learns to seize upon those 

 resemblances which are essential to his classification, and to 

 cast from his thoughts those differences which are unessential." 

 This is certainly a royal road to geology, a clue to the laby- 

 rinths of geological theories. 



But to return. If examples of Cornish slates be disliked, 

 take hand-specimens of the greywacke roofing-slates of Cum- 

 berland and Wales. Are these true slates? Undoubtedly not, 

 according to the new system, for we are informed (at p. 474.) 

 that " these thin laminae often resemble the coarser varieties 

 of slate, and indeed are sometimes used for the same purposes, 

 but they are only flagstones or thin beds." How is this 

 determined? By the same rule as denied to the Cornish 

 schists the privilege of being slates, viz. parallelism to the strata; 

 for which reason, it is also said (at p. 479.) that " the oldest 

 and most crystalline rocks, designated by the general name 

 of schists, have no true slaty cleavage, in the sense in which 

 I have used the term. The value of this rule may be better 

 appreciated, as conferring a " definite meaning" on the term 

 true slaty cleavage, when it is remembered that the Professor 

 will not allow the word stratum to be susceptible of an accu- 

 rate definition. 



[To be continued.] 



