ANNIVEESAEY ADDEESS OF THE PEESIDENT. XXXV 



on the separation of the several parts this combination — water— has 

 been formed, and set apart dui^ing the solidification of the crystals, 

 especiallj^ of c^nartz. The rock has not been formed by or in water, 

 but water has been formed in the rock. The presence and abun- 

 flance of oxygen has been long known ; Avhat the enclosed liquid 

 tells us further is the presence of hydrogen. 



Other liquids besides water occur. In one case, of amethj'st, the 

 cavity being three-fourths full of liquid at ordinary temperature, 

 becomes full of liquid at 83° F. ; on being cooled again the vacuity 

 reappears in the crystal, with signs of ebulhtion*. 



Naphtha and bitumen occur in other crystals. 



Perhaps hardly anj'thing is more essential to a good classification 

 of igneous and metamoi^oliic rocks than a correct notion of the 

 minerals known to geologists as felspar ; for this is now rather a 

 large family of varying chemical compounds whose crystalHne struc- 

 ture includes several types f. In a general sense, granite usually 

 contains orthoclase-felspar, with potash ; but there are cases where 

 one of the constituents is albite or soda-felspar ; and it appears that 

 an orthoclasic crystal may have soda in its composition, We are 

 indebted to Prof. Haughton and Mr. Galbraith for the most con- 

 tinued observations on this class of subjects in, as well as on the 

 varieties of mica in the granites of, Ireland. The microscope, with 

 polarized light and other optical means, Avill probably be usefully 

 employed in the anorthic felspathic minerals which are so common 

 in diorites and greenstones. 



The arrangement of the materials which compose a stratified rock 

 is one of the subjects for which the British strata offer many advan- 

 tages, and promises of important results. By this study well prose^ 

 cuted, we may distinguish between sands drifted by wind or de- 

 posited by water,- — the rapid accumulations along the sea-shore, 

 or the slow subsidence of finer sediments in deeper water, to which 

 Mr. Babbago has lately called our attention J. We may determine 

 the direction of sea- currents, conjecture theii' form, extent, and 

 origin, and the situation and physical character of the region whence 

 rivers flowed into estuaries, or from the shores of which the waves 

 transported detritus from old deposits to form new strata in other 

 situations. 



No one has been more successful than Mr. Sorby in observing 

 phaenomena of this kind, or more ingenious in applying to them 

 interpretations in accordance Avith the laws which are known to 

 govern the operations of moving water. In the Oolitic tracts of 

 Gloucestershire, in, the arenaceous deposits of the same age, and of 

 the Magnesian and Older Carboniferous period in Yorkshire, the 

 strata are often full of obUqiie lamination, due to shallow ciuTcnts 

 of water operating on easily moved sand, comminuted shells and 

 corals, or other small bodies. By a careful study of these charac- 



* Sir D. Brewster, Ed. Roy. Soc. Trans., and Ed. Phil. Journ. ix. 

 t See the classification of felspathic minerals by the ratios of the oxygen in 

 their component elements, by M. Ste.-Clau'e Deville, in his ' Etudes de Lithologie.' 

 I Quart. Joiu'n. Qeol. Soc. vol. xii. p. 366. 



