Xviii THIRD REPORT — 1833. 



legist change from one moment to another. Another difficulty 

 is this ; that while we want to determine what takes place in 

 the whole depth of the aerial ocean, our observations are neces- 

 sarily made almost solely at its bottom. Our access to the 

 heights of the atmosphere is more limited, in comparison with 

 what we wish to observe, than our access to the depths of the 

 earth. 



" Geology, indeed, is a most signal and animating instance of 

 what may be effected by continued labours governed by common 

 views. Mr. Conybeare's Report upon this science gives you 

 a view of what has been done in it during the last twenty years ; 

 and his ' Section of Europe from the North of Scotland to the 

 Adriatic,' which is annexed to the Report, conveys the general 

 views with regard to the structure of Central Europe, at which 

 geologists have now arrived. To point out any more recent 

 additions to its progress or its prospects is an undertaking 

 more suitable to the geologists by profession, than to the pre- 

 sent sketch. And all who take an interest in the subject will 

 rejoice that the constitution and practice of the Geological So- 

 ciety very happily provide, by the annual addresses of its Pre- 

 sidents, against any arrear in the incorporation of fresh acquisi- 

 tions with its accumulated treasures. 



" The science of Mineralogy, on which I had the honour of 

 offering a Report to the Association, was formerly looked upon 

 as a subordinate portion of Geology. It may, however, now be 

 most usefully considered as a science co-ordinate and closely 

 allied with Chemistry, and the most important questions for 

 examination in the one science belong almost equally to the 

 other. Mr. Johnston, in his Report on Chemical Science, has, 

 as the subject required, dwelt upon the questions of isomor- 

 phism and plesiomorphism, which I had noticed as of great im- 

 portance to Mineralogy. Dr. Turner and Prof. Miller, who at 

 the last Meeting undertook to inquire into this subject, have 

 examined a number of cases, and obtained some valuable facts ; 

 but the progress of our knowledge here necessarily requires 

 time, since the most delicate chemical analysis and the exact 

 measurement of 30 or 40 crystals are wanted for the satisfac- 

 tory estabhshment of the properties of each species *. In Che- 



• Perhaps I shall not have a more favourable occasion than the present of 

 correcting a statement in my Report, which is not perfectly accurate, on a point 

 which has been a subject of controversy between Sir David Brewster and Mr. 

 Brooke. I have noticed (p. 338.) the sulphato-tricarbonate of lead of Mr. 

 Brooke, as a mineral which at first appeared to contradict Sir David Brewster's 

 general law of the connexion of crystalline form with optical structure, in as 

 much as it appeared to be of the rhombohedral system, and was found to have 



