^^4 Baron Humboldt on Rock Formations, 
age, Sir Humphry Davy, has turned his attention to it, and 
discovered that the gases, at a high pressure, are powerfully 
acted upon by slight increments of temperature, and the pressure 
astonishingly augmented. 
This circumstance affords us flattering hopes that an engine, 
constructed on the principles of the Reverend Mr Stirling’s air- 
engine, may yet be made to equal, and in many cases to super- 
sede, the steam-engine, by reducing the expenditure of sulphuric 
acid. As I have had numerous letters from various parts of the 
kingdom, requesting a description of the soda-water-apparatus, 
your giving this a place in your valuable Journal, will, I trust, 
be a sufficient answer to them, and will confer a very high favour 
on the author. 
Glasgow, ) Charles Cameron. 
December 10. 18S3. f 
Art. V . — On Rock Formations. By Baron Alexander 
Hhmboldt. (Concluded from p. 53.) 
JF bom that scepticism which would deny the existence of any 
kind of regular order in the position of rocks, it is proper to dis- 
tinguish an opinion which has sometimes found supporters 
among experienced observers. According to this opinion, the for- 
mations of gneiss-granite, of greywacke, of alpine limestone, 
and of chalk, Avhich have a uniform superposition in different 
countries, do not very well correspond among themselves as to 
the age of the homonymous elements of each series. It is thought 
that a. secondary rock may have been formed on one spot of the 
globe, while transition rocks did not yet exist on another spot. 
In this supposition, no allusion is had to those granitic rocks 
which are found lying above limestone containing orthoceratites, 
and which are consequently newer than the primitive rocks. It 
is a fact generally admitted at this day, that formations of ana- 
logous composition have been repeatedly deposited at epochs far 
removed from each other. The doubt which we are no\7 consi- 
dering, (though we do not partake in it), bears on a point much 
less clearly established, — the ascertaining whether certain mica- 
slate rocks, evidently situate in the midst of a country of primh 
