286 PKOF. T. G. BONNET ON MESOZOIC EOCKS AND [Aug. 1 894, 



ground, and has supplied me with much valuable information. The 

 results of my work, I regret to say, are far from being decisive, and 

 my only excuse for laying them before the Society is the exceptional 

 interest of the question, and the hope that they may be useful as a 

 record of facts of which account must be taken in framing any 

 hypothesis. 



But before proceeding further it is almost necessary, in order to 

 dissipate a confusion which I have perceived to exist, to repeat what 

 has been already stated in print, so far as to make clear the exact 

 points at issue between myself and these eminent foreign geologists. 



(1) I have never denied that Jurassic rocks form a part of the 

 sedimentary belt in which the Altkirche marble occurs — I know the 

 Alps too well to do anything of the kind. Nor do I deny that the 

 stratigraphical evidence seems at first sight favourable to regarding 

 the marble as merely a peculiar member of the group of Jurassic 

 rocks. My position is, that such an identification proves, on a more 

 careful scrutiny of the sections, to be beset with difficulties, while, 

 so far from receiving any support from, it is contradicted by other 

 regions of the Alps, where the sections are clearer. 



(2) I have never denied that in the Alpine chain the sedimentary 

 rocks, to say nothing of the igneous, have undergone, in consequence 

 of the mechanical disturbances to which they have been subjected, a 

 certain amount of structural and of mineralogical change, and might 

 thus be termed ' metamorphic ' rocks 1 • but I have affirmed, and I 

 now do it yet more emphatically, that the results of these changes 

 generally can be recognized, and are not comparable, in the case of the 

 later Palaeozoic or Mesozoic sediments of the Alps, with the alterations 

 which, anterior to these disturbances, have converted into crystalline 

 schists certain sediments of unknown antiquity. Hence I consider 

 it better, if it be desired to avoid confusion of expression and thought, 

 either to abstain from applying the term ' metamorphic ' to the former 

 results or to devise a new connotation for the latter. 



(3) It is also necessary to repeat (strange as this may seem) tbat 

 a transitional passage of a sedimentary into a crystalline rock cannot 

 be inferred from the existence of a comparatively narrow intermediate 

 zone in which the destructive effects of pressure have been so great 

 as to make it doubtful whether this represents a crushed condition 

 of the crystalline rock, with slight secondary change, or a squeezed 

 condition of a clastic rock (especially if the fragments be derived 

 from the crystalline one) with a similar change. Neither can 

 identities be inferred from superficial resemblances, unless a micro- 

 scopic study shows these to depend upon a real community of 

 structure and composition. 



1 It must be remembered that certain constituents of rocks are more ready to 

 change than others. For instance, carbonate of lime very readily crystallizes. 

 Hence, in dealing with an apparently crystalline limestone, it is necessary to 

 study the rock as a whole, and to pay great attention to the condition of other 

 constituents. Silicates are more useful, as a rule, than quartz, for grains of the 

 latter readily become enlarged and thus are apt to lose their clastic characters. 



