SORBY STRUCTURE OF CRYSTALS. 497 



veins. It also penetrated into the stratified rocks, heated, some- 

 times for a great thickness, to a high temperature, and assisted in 

 Changing their physical and chemical characters, whilst that re- 

 maining amongst the partially-melted igneous rock served to modify 

 the crystalline processes which took place during its consolidation. 

 These results are all derived from the study of the microscopical 

 structure of the crystals ; but my own observations in the field lead 

 me to conclude that they agree equally well with the general struc- 

 ture of the mountains themselves, and serve to account for facts that 

 could not have been satisfactorily explained without the aid of the 

 microscope. And here I cannot but make a few remarks in conclu- 

 sion on the value of that instrument, and of the most accurate 

 physics in the study of physical geology. Although with a first- 

 rate microscope, having an achromatic condenser, the structure of 

 such crystals and sections of rocks and minerals as I have prepared 

 for myself with very great care can be seen by good day-light as 

 distinctly as if visible to the naked eye, still some geologists, only 

 accustomed to examine large masses in the field, may perhaps be 

 disposed to question the value of the facts I have described, and to 

 think the objects so minute as to be quite beneath their notice, and 

 that all attempts at accurate calculations from such small data are 

 quite inadmissible. What other science, however, has prospered by 

 adopting such a creed ? What physiologist would think of ignoring 

 all the invaluable discoveries that have been made in his science 

 with the microscope, merely because the objects are minute 1 What 

 would become of astronomy if everything was stripped from it that 

 could not be deduced by rough calculation from observations made 

 without telescopes? With such striking examples before us, shall 

 we physical geologists maintain that only rough and imperfect 

 methods of research are applicable to our own science 1 Against 

 such an opinion I certainly must protest ; and I argue that there is 

 no necessary connexion between the size of an object and the value 

 of a fact, and that, though the objects I have described are minute, 

 the conclusions to be derived from the facts are great. 



DESCRIPTION OF PLATES XVI. to XIX. 



The number of times that the objects are magnified in linear dimensions is 

 expressed by the sign X . The figures with a dotted outline are portions of 

 crystals, and the rest are entire separate cavities and crystals. 



Plate XVI. 



Cavities if Ceystals formed Artificially. 

 § 1. From solution in Water. 



Fig. 1. A single crystal of chloride of potassium deposited on slow evaporation 

 in winter. X 60. 



Fig. 2. A portion of the edge of a crystal of chloride of sodium, X 200, showing 

 how the fluid- cavities are formed. 



Fig. 3. A fluid-cavity in chloride of potassium formed at the ordinary tempera- 

 ture. X800. 



Figs. 4, 5. Fluid- cavities in chloride of sodium formed at the ordinary tempe- 

 rature. 4, X800; 5, XlOOO. 



