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186 



GEOLOGY. 



[Sect. VI. 



gec^. 





of which the common direction ought to be recorded • 

 how far the mountains stand apart, and the diameter and 

 outline of the rude circle which they together form. In 

 fact, a most useful service would be rendered by mappino* 

 any of these "craters of elevation/' or, what would be 



more feasible, drawing from actual measurements two 

 sections at right angles to each other, across the circle. 



Some streams of lava, especially those belonging to the 

 trachytic series (harsh, generally rather pale-coloured 

 lavas, with crystals of glassy feldspar), are laminated. 

 The course of the layers with respect to the course of the 

 stream ought to be minutely studied, both on tlie surface, 

 at the termination, and flanks of the stream ; and, if by 

 a mo^t fortunate chance there should have been formed 

 a transverse section, throughout its entire thickness : this 

 would be a very interesting subject for investigation. A 

 series of specimens ought to be brought away to illustrate 

 the nature of the lamination. 



Aerial Dust, — Fine brow^n-coloured dust has often 



fallen on vessels far out at sea, more especially in the 

 middle of the Atlantic. This should be collected ; the 

 direction and force of the wind (and the course of any 

 upper current, as shown by the movement of the clouds) 

 on the same day, and for some previous days, ought to 

 be recorded, as well as the date, and the position of the 

 ship. Such dust has been shown by Ehrenberg to con- 

 entirely of the siliceous 

 envelopes of infusoria. The distance to which real vol- 



sist, in many cases, almost 



canic dust is blown is, likewise, in some respects well 



worth determining. 



Elevation of the Land. — The changes of level, often 



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