ANNIVEKSAKY ADDRESS OF THE PKESLDENT. 99 



look, although evidently, in the case of the former, much recrys- 

 tallization, in situ, has taken place, so that the rock, at first sight, 

 has quite the appearance of a normal crystalline schist. A white 

 mica is here fairly abundant. The mica flakes are very often 

 parallel with the directions of the thicker bands ; but even in these 

 the rule is by no means universal, while the thinner layers some- 

 times exhibit much irregularity, a majority of the flakes occasionally 

 pointing in the direction of the layer. This is also true of the most 

 cleavable specimens, in which, even in the thicker bands, the dominant 

 tendency of the mica flakes is rather in the general direction of the 

 thinner bauds, viz. at a considerable angle with the former. 



These structures, as it appears to me, can only be explained as 

 follows : — The rock must have been already a crystalline foliated 

 rock before it was exposed to the pressure to which the present 

 modifications of structure are due. When this acted, it must have 

 been in a condition (probably of slight plasticity) which allowed of 

 a certain amount of movement of its constituents without serious 

 rupture of continuity*. Thus the folia were bent, and the mica 

 flakes, probably with some amount of recrystallization, were en- 

 abled to arrange themselves at right angles to the pressure (fig. 4). 

 In the other cases, whether from slight diff'erences in the constitution 

 of the rock, or from local circumstances causing it to yield more 

 readily, there was more disturbance of the constituents, the mica 

 layers were more distorted, and in some cases were pressed or drawn 

 together (fig. 3). Here also a certain amount of recrystallization, 

 how much it is difficult to say, has taken placet. 



It will be noticed that my explanation of these structures does 

 not quite agree with that given by my distinguished predecessor 

 Dr. Sorby. It is with the utmost difiidence that I differ even in 

 the slightest degree from so careful an observer and so acute a 

 reasoner, but I have been unable to come to any other conclusion. 

 He regards, if I understand him rightly, the cleavage-foliation as 

 the result of the metamorphism of a cleaved detrital rock, while I 

 regard it as a structure superinduced upon a rock which was already 

 possessed of stratification-foliation. 



Further to the south, in the cliffs a little north of Stonehaven, the 

 cleavage-foliation has practically obliterated the stratification-foli- 

 ation. The rock also, as already said, has a much more marked 

 fragmental character, so that many specimens might be readily mis- 

 taken for slightly metamorphosed clastic rocks. Here and there, 

 however, I can detect remnants of the old lines of foliation, which by 

 their pressing or dragging together have formed the now dominant 

 bandings of mica which make the cleavage-foliation. It is inter- 

 esting to note that in these rocks the mica flakes are generally 

 much less perfectly defined than in those described at Muchails, 

 the mineral being, what I may call, confusedly crystallized. It is 

 almost impossible to say precisely what is the origin of some of 



* The possibility of this is indicated by the well-kuown experiments of 

 Tresca and others on the "flow of sohds." 



t I may remark that not seldom a mica, practically colourless in thin 

 sections, seems to form from biotite by the extrusion of the iron-oxide. 



