ANNIVEKSARY ADDRESS OF THE PRESIDENT. Ixxvii 



Geology, and on Microscopic Studies of Rocks*. Tlio volume is, 

 in three simple words, " England's Geologen gewidmct," dedicated 

 to England's geologists, and, after a first division dealing with the 

 auxiliary sciences, devotes a second to the history of the develop- 

 ment of geology, in which our British chiefs in geological theoiy 

 play a very prominent part. His review of the series of leading 

 authors and philosophers, from the time of George Agricola to our own 

 day, is couched in a bright and vivacious style ; and an unusual ori- 

 ginality and independence of judgment are shoAvn in the various 

 degrees of merit allotted to the great names of the science. The 

 third division of the book opens with the newer phases of geology, 

 and, ascribing to our valued associate Mr. H. C. Sorby the full 

 credit of introducing so important a step in the development of 

 petrography, enters upon the microscopical examination of a series 

 of sliced rock-substances, selected in a great measure for their rela- 

 tion to various moot points in theory. 



In opposition to certain recent observers who assert that by the 

 aid of the microscope they have been able to resolve the whole of a 

 porphyry, including its "paste" (Grundmasse), into a compound 

 of recognizable crystals. Dr. Yogelsang insists that, just as nebulous 

 matter in the heavens has proved irresolvable under the highest 

 power of the most powerful telescopes, so the paste of a great number 

 of the porphyries is a decidedly un crystallized mass, and, further, 

 that the form of the cavities, and the position of the minute crystals 

 or microlites contained in it, testify to the mechanical action of the 

 movement of a more or less Hquid substance. 



Whilst agreeing with Mr. Sorby in the facts of observations upon 

 the fluid-cavities or water-pores contained in the quartz of quartzi- 

 ferous silicate rocks, he draws a different deduction from their 

 presence. It will be recollected that these microscopic cavities are 

 only partially filled with liquid, and that, from the relative size of 

 the bubble, Mr. Sorby suggested that conclusions might be drawn 

 as to the temperature at which the mass had solidified. Dr. Vogel- 

 sang finds that the ratio of the bubble to the cavity is not constant 

 in the same rock, or even in the same crystals, and holds that the 

 fluid has been introduced by secondary action into the cavities. 

 Mr. Sorby, in his original paper, read before the Society in December 

 1857, had not omitted to discuss this alternative, especially with 

 reference to the fluid- cavities in the nepheline of Monte Somma, 

 and in the quartz of granite and elvan ; and he then alleged such 

 good reasons for doubting any other explanation than that of the 

 fluid having been enclosed at the time of formation of the mineral, 

 that we shaU. need further evidence to lend support to an opposite 

 view. The issue of the question will awake much interest when it 

 is recollected how ingeniously Mr. Sorby deduced from his micro- 

 scopic vacuities, among other things, the inference that the granites 

 of Cornwall and Aberdeen were consolidated under pressures varying 

 from 50,000 to 78,000 feet of rock. 



* Philosophie der Geologie und mikroskopische Gesteinsstudien, von Dr. H. 

 Vogelsang, Professor zu Delft. Bonn, 1867. 



