722 PROCEEDINGS OF THE SOCIETY. 



was made by Dr. George Jackson, the optical portion, object-glasses, 

 and eye-pieces were the work of Mr. James Smith. The Society was 

 greatly indebted to Mr. John Jackson for this very interesting present. 



Dr. Hebb said the Microscope which had been presented to the 

 Society by Mr. Waters and his sister, Miss Celia Waters, was made by 

 Cary, and was believed by Mr. Waters to be at least 60 years old. It 

 was fitted with Varley and Sons' Lever Stage, the upper plate of which 

 bad its movement controlled by an ingenious parallel motion placed 

 below it. A Varley Microscope, having a stage constructed on this 

 principle, is figured in QueJcett on the Microscope, 1848. 



Messrs. Elliott Bros, sent a Class Microscope for exhibition, which 

 was passed round for the inspection of the Fellows present. 



On the Motion of the President, a cordial vote of thanks was passed 

 to the donors of these instruments and of the other presents to the 

 Society, and to Messrs. Elliott Bros, for their exhibit. 



The President said they were to be favoured that evening with what 

 he was sure would be a most interesting demonstration by Prof. Bonney 

 1 On Rock Changes in Nature's Laboratory,' and the Society was much 

 indebted to Prof. Bonney for giving them his valuable time on that 

 occasion. 



Prof. Bonney said he would commence his remarks by a few ex- 

 planatory words, because he thought that the subject upon which he was 

 going to speak might perhaps lie a little outside the range of those 

 which usually came under the notice of the Society. The changes 

 which took place in nature were brought about by the operation of three 

 great agencies, water, pressure, and heat, only here their action went on 

 much more slowly than any process which we were obliged to adopt, 

 Nature not being cramped for time or limited to threescore years and 

 ten. We could apply heat, for instance, and could melt a portion of 

 rock, and by its fusion produce a glass, but Nature, working slowly, 

 could crystallise these substances in a way which we could not. The 

 three forces seldom acted entirely apart from each other, often two, and 

 sometimes all three, working together. For example, a layer of ordinary 

 mud depressed to a considerable depth, and covered up with similar 

 material, is subjected to pressure. The weight of about 12 ft. of 

 average rock is roughly that of one atmosphere, whilst the earth tem- 

 perature is known to increase at the rate of about 1° for every 60 ft. 

 of descent. So that a layer of Thames mud when buried under 3000 ft. 

 of similar material would be subject to a pressure of 250 atmospheres, 

 and have its temperature raised 50°, or twice its present average 

 amount. Commonly, however, some one of these forces was dominant, 

 but in other cases each has dominated in its turn. In the district of 

 Skiddaw, for instance, the slaty rock has been heated by intrusive 

 granite, but at Glendalough, in Wicklow, the granite came first, and 

 then pressure acted and forced the altered rock to assume a slaty con- 

 dition. Tho subject he had chosen was a rather wide one, and he there- 



