ARCHAEAN. 



37 



weakness) along which decomposition goes on most readily. The principal 

 of these solution planes is the basal plane, and parallel to it we find the 

 gems eaten away in a series of step-like surfaces. Other less pronounced 

 planes of chemical weakness exist parallel to the prism faces. Unaltered 

 corundum is, like quartz, destitute of true cleavage, and breaks with a per- 

 fectly conchoidal fracture. If, however, gliding planes and lamellar twinning 

 be developed in corundum (like those so easily produced in the same way in 

 calcite), parallel to the fundamental rhombohedron of the crystals, then these 

 gliding planes become ' solution planes,' along which chemical action takes 

 place most readily. Along the primary or secondary solution planes, hydra- 

 tion of the aluminium oxide takes place, and disapore is formed, as shown by 

 Lawrence Smith and Genth, and this unstable mineral enters into combina- 

 tion with silica and other oxides present to give rise to the numerous pseudo- 

 morphs of corundum, which are so well known to mineralogists." 



In the paper published in extenso in the Philosophical Transac- 

 tions, Professor Jiidd gives further details regarding the supposed origin 

 of the limestone from the alteration of the unstable scapolite con- 

 tained in the basic gneisses, 1 and it must be said that, given no 

 possibility of the mass of the limestone having been of sedimentary 

 origin, the theory put forward seems to be quite convincing. But, 

 diffident as I feel in criticising the conclusions of so high an author- 

 ity as Professor Judd, I must say that the result of my own 

 observations in the field has led me to entertain considerable doubt 

 as to the adequacy of the explanation given to account for the 

 formation of such enormous masses of practically pure calcite. 

 The manner in which these bands of limestone occur in the field 

 is so much in agreement with their having been originally com- 

 ponents of a sedimentary series, associated along a particular zone 

 with rocks of igneous origin (whether intrusive or contemporary 

 cannot now be determined, owing to the intense pressure and dis- 

 location to which the whole complex has been subjected), that it 

 would require, to my mind, the very clearest evidence to show 

 that they could never have been sedimentary rocks. As Mr. 

 Barrington Brown has shown, 2 they occur only along a particular 

 zone in the gneisses, and within this zone they appear to have 

 consisted of several continuous bands, which have been broken up 

 and drawn out by earth stresses in such a manner that they now 

 present the appearance of a number of greatly elongated, lenticular 

 masses, each individual band dying out completely at intervals, 

 but continued en echelon across the country ; much as veins of 



Loe. cit., p. 214. 

 Loc. cit., p. 150 scq. 



