Sect. VI.] 



GEOLOGY. 



185 



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making ^^ich observations, comparatively recent streams 

 must be chosen, so that there can be no donbt that the 

 whole consists of a single stream : this cannot be judged of 

 without examining the whole line between the two points 

 of measurement, forborne liquid lavas thin out to a very 

 fine edge ; and tv/o streams, one over the other, may be 

 thus very easily mistaken for a single one. The compo 

 sition, thickness, and degree of cellularity of any lava 



stream 



s 



of which the s 



lope is measured, ought to be 



described as seen on the sides of fissures, and wherever 

 its internal structure can be made out. 



Round manv active and extinct volcanoes, both on con 

 tinents and on islands, there is a circle of mountains. 



steep on their inner, and gently inclined on their outer 



flanks. The volcanic strata, of which they are composed, 

 everywhere dip away from the central space, but at a 

 considerably higher angrle than it is believed lava can 

 consoUdate into such thick and compact masses, 

 mountains form the so-called ^^ craters of elevation,'' the 

 origin of which has excited much controversy, and which 

 demand further examination. There is a grand range of 

 mountains of this class at the Mauritius and at St. Jago 

 in the Cu.pe de Verdes, parts only of which have been 





described. The chief points to attend to are, th-^ '--''-^ 



e mclina 



tion of the streams by actual measurement, their thick- 

 ness, compactness, a.nd composition ; the form and height 

 of the mountains, whether traversed by very many dikes, 



image of the black line, 1 have only to make the image of any object 

 reflected from the moveable mirror coincide with that of the block line, 

 to have the -^^ gle bet^* en the vertical, and the line drawn from the centre 

 of the instrument to the object in question, which may be any distant point 

 on the surface of a bed of lava, a glacier, a road, a river, occ/' 



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