Considerations on Volcanoes. 365 



friction of the lava on the sides of such fissures, its grain must have 

 been comminuted ; by which means we must explain the greater 

 degree of fineness which these possess at their sides. Currents of 

 lava " progress" even after the original vent is closed. And 

 sometimes the lava escapes from the upper crust, leaving a ca- 

 vern, as happens in Iceland. But we must here refer to the author 

 himself for a variety of curious but well-known particulars, which 

 do not well bear abridgment. We prefer selecting what is new 

 whenever we can find it. 



And, therefore, we will here notice that, as the crystals or parts 

 of lava in a current proceed by a " sliding and slipping motion of 

 their plane surfaces over each other, facilitated by the interven- 

 tion of the elastic fluid, rather than by the rotatory movement 

 which actuates the globular molecules of most other liquids," the 

 crystals of trachyte will arrange themselves in the currents, so as 

 to have their longer axis in the direction of the motion. Thus a 

 very small quantity of vapour interposed between the parallel 

 surfaces of proximate crystals will allow them to glide past each 

 other, while the motion in a lateral direction will become propor- 

 tionately difficult. We felt it necessary to give this view thus 

 clearly, because we think that Dolomieu and Kirwan had both 

 failed in explaining the cold fluidity of lavas, particularly when 

 at the white heat formerly noticed. 



This also, it is important to remark, accounts " for the extreme 

 difficulty with which a stream of lava is induced to alter the di- 

 rection it has been once led to assume; as it also does for the 

 great perfection of the angles of the crystals ; a fact which those 

 two philosophers had equally failed in explaining." In this case, 

 the quantity of vapour which communicated the " sliding or 

 glissant motion" in one direction, may be insufficient to act 

 when the position of the crystals is unfavourable ; and the friction 

 from obstacles, explains also why the particles and crystals are 

 finer at the extremity of a current than higher up, or in the middle ; 

 a circumstance which others had attempted to explain by imagin- 

 ing a more slow and more perfect crystallization as the necessary 

 consequence of greater fluidity and a longer period of cooling. 



The consolidation of lava occupies the fifth chapter. 



Lest the author, and we after him, had not clearly explained the 

 state of lava in fusion already, it is repeated here once more, and 

 we shall, therefore, give the explanation. When the component 

 crystals of lava have been disintegrated by the vapour, part of it 

 still remains " imprisoned between the crystalline laminae," cre- 

 ating, by its elasticity, a sort of repulsion between them, without 

 altering the positions of their axes, the remainder being free, or 

 having liberated itself from the points on which it was first va- 

 porized by displacing the laminae and disintegrating the crystal, 

 the parts becoming smaller crystals are conformably disposed 



Vol. XX. 2 C 



