364 Analysis of Scientific Bodks. 



considered as four to five ; and all other circumstances being 

 equal, it is presumed that the fluidity will be proportioned to the" 

 specific gravity. 



The force of expulsion by which lava is projected from the vent 

 is called the ratio of production, " varying directly with the ra- 

 pidity of the internal progress of expansion through the focus, 

 i. e., with the energy of eruption." And " if this term be fixed, 

 it varies inversely with the fluidity of the lava, since the degree 

 of intumescence by which it is occasioned preserves this propor- 

 tion." We use the author's own words, that we may convey an 

 idea of the mathematical precision of his language, so superior to 

 any that we could produce. 



Thus again : " The external circumstances that affect the dis- 

 position of lava upon the earth's surface, are divisible into, first, 

 those which tend to diminish its fluidity; and secondly, those 

 which favour or impede the lateral extension to which it is urged 

 by that fluidity." 



Thus, if it is fluid it flows, at least if there are no obstacles to 

 stop it, " until the cessation of the force of expulsion and the 

 progress of consolidation in the mass already expelled gradually 

 diminish its velocity, and finally arrest its further progress." 

 Thus it assumes on cooling various forms, and becomes a sheet, a 

 nappe, or a plateau, or a stream, or a current, or a coulee, or a 

 hummock, or a dome. 



If it contains much iron, it has a tendency to spread in sheets, 

 and if the reverse, producing trachyte, it forms domes. And here, 

 the author observes, that geologists have committed a serious 

 error in asserting that the trachytic rocks had not flowed on the 

 surface, but had been elevated in the forms in which they are now 

 found ; equally controverting Von Buch and Humboldt, who affirm 

 that they had been blown up like bladders. Trachyte is, however, 

 found also in sheets ; but these are always thick, and have a ten- 

 dency to form hummocks. 



The author having again reminded us, that " the quantity and 

 escaping force of the vapours evolved from lava in ebullition 

 varies directly, and the quantity of lava protruded en masse by 

 its internal intumescence, inversely with its fluidity," concludes, 

 among other things, that the trachytic rocks have been elevated 

 by the same causes and in the same manner as basalt, and that 

 the difference of their bulk arises from a lower degree of fluidity 

 arising from an inferior specific gravity. 



Here, at page 100, follows a very minute description of the 

 " mode of procedure of lavas in general when poured out upon 

 the earth's surface." Veins or dikes, such as those of the North 

 of Ireland, he considers, have been formed from such currents de- 

 scending into previous fissures, in some cases, while in others they 

 have been the volcanic vents. And he considers also, that by the 



