ANNIVERSARY ADDRESS OF THE PRESIDENT. CXXV11 



sophical conclusions of high, interest. Mr. Sorby' s application of the 

 results of his inquiries to the explanation of the phenomena of clea- 

 vage is fresh in our recollection ; and he has since pointed out the 

 misapprehension of his views which even Professor Tyndall has 

 shared with many others. Mr. Sorby intended to show that the 

 compressive force, which he considers essential for the production 

 of cleavage, acting upon the unequiaxial particles of mica, would lead 

 to their arrangement in planes corresponding to those of the induced 

 cleavage, and, consequently, that the cleavage in slate-rocks is more 

 perfect in proportion as the quantity of mica present increases ; and he 

 further states, " there are scarcely any rocks whose particles are not 

 unequiaxed, and I must still maintain that, other circumstances 

 being the same, those have the best cleavage that are composed of 

 particles whose length and thickness differ most." It is therefore 

 not as being the primary cause of cleavage, but as influencing its de- 

 gree of perfection, that Mr. Sorby cited the mode of arrangement of 

 the particles of mica in slate and other rocks ; and few will doubt 

 that so far his reasoning is correct, as the planes of crystallization 

 will naturally become coincident with the planes of cleavage. In 

 his paper on the " Limestones of Devonshire," Mr. Sorby, in a 

 similar manner, endeavours to show that the unequiaxed particles 

 have by compression been thrown into planes, perpendicular to the 

 direction of pressure, which have become the points or spaces of least 

 resistance of Professor Tyndall, or those in which, according to my 

 view of the case, the reaction to pressure exceeds the cohesive force, 

 the planes in fact of least cohesion. In another paper, also of recent 

 date, Mr. Sorby endeavours to prove that the terraces in the Yalley 

 of the Tay, north of Dunkeld, have been " formed by the combined 

 action of the river and of the sea when it was at a relatively higher 

 level ;" and this idea he ingeniously supports by showing that the 

 stratula of the bands of ripple-drift, or drift-bedding, dip on the whole 

 along a mean line to one side, being that to which the current flows 

 or in which the directing pressure is applied, just as in his previous 

 experiments in the preceding paper, — a current therefore which alter- 

 nately moves in two directions naturally giving rise to successive 

 opposite dips of the stratula. Having ably maintained his views, 

 Mr. Sorby judiciously adds, " it must not be supposed that I wish 

 to make it appear that the terraces in all other valleys are due 

 to the same cause, — as one set of circumstances may have formed 

 some, and another set others. Nothing, in my opinion," he adds, 

 " can be a greater obstacle to a correct interpretation of such phe- 

 nomena, than to conclude that all things which appear similar are 

 actually identical and have had a similar origin," — remarks, in the 

 truth of which I fully concur. 



In the papers of Mr. Sorby which I have been noticing, the mi- 

 croscope was applied to the elucidation of the manner in which the 

 constituent particles of rocks had been arranged, and thereby to the 

 explanation of the true character of some important physical pheno- 

 mena. In the paper which Mr. Sorby contributed to our proceedings 

 during the present session, the same instrument has been applied to 



