A HISTORY OF SCIENCE 



" But as the torrents of vapor, holding these various 

 matters in solution and suspension, were forced up- 

 ward, the greater part of the disintegrated crystals 

 by degrees subsided ; those of felspar and quartz first, 

 the mica being, as observed above, from the form of 

 its plates, of peculiar buoyancy, and therefore held 

 longest in suspension. 



"The crystals of felspar and quartz as they sub- 

 sided, together with a small proportion of mica, would 

 naturally arrange themselves so as to have their long- 

 est dimensions more or less parallel to the surface on 

 which they rest; and this parallelism would be subse- 

 quently increased, as we shall see hereafter, by the 

 pressure of these beds sustained between the weight 

 of the supported column of matter and the expansive 

 force beneath them. These beds I conceive, when 

 consolidated, to constitute the gneiss formation. 



"The farther the process of expansion proceeded in 

 depth, the more was the column of liquid matter 

 lengthened, which, gravitating towards the centre of 

 the globe, tended to check any further expansion. 

 It is, therefore, obvious that after the globe settled 

 into its actual orbit, and thenceforward lost little of 

 its enveloping matter, the whole of which began from 

 that moment to gravitate towards its centre, the prog- 

 ress of expansion inwardly would continually increase 

 in rapidity; and a moment must have at length ar- 

 rived when the forces of expansion and repression had 

 reached an equilibrium and the process was stopped 

 from progressing farther inwardly by the great pressure 

 of the gravitating column of liquid. 



"This column may be considered as consisting of 



134 



