118 VEINS OF LAVA, OR DYKES. 



even on steep slopes, as at Teneriffe. On the contrary, stony iavas 

 which cool slowly and long remain fluid, are not arrested except 

 on a horizontal plain. 



35. Various characters of the same lava. From what has been 

 stated, it is certain that lavas cannot accumulate to a great thick- 

 ness, or spread in sheets, except on a horizontal plain. The struc- 

 ture of lava depends, in a degree, on its external arrangement. The 

 vein, which is behind -the current, on a very steep slope, is, in parts, 

 thin, scoriaceous, corded, and always very porous. On less steep 

 slopes, the surface of pieces is more united, the pores are smaller ; 

 on descents, at an angle of from three to five degrees, the dislocated 

 parts are in plates of greater or less thickness, the structure of 

 which presents a certain uniformity, and the centre is sometimes a 

 little more compact, if the thickness is sufficient. In great flows, 

 causing great accumulations on plains, where the depressions are 

 filled up, all the inferior part becomes a compact, and, more or 

 less, crystalline mass, which is porphyritic, because then it cools 

 slowly and tranquilly ; in this case it is frequently divided, through 

 its whole height, into columnar masses, generally normal on the 

 cooling surfaces, and porous at the upper part only; this is seen 

 at Vesuvius and Etna, where the lava is very thick, and at Iceland 

 in the immense deposit formed by the eruption of 1783. 



36. Feins of Lava, or Dykes. It frequently happens, that in 

 volcanic eruptions there is formed, on the sides of the mountain, 

 crevices of greater or Jess breadth, through which the lava comes 

 to the surface of the soil. These cracks are remarked for a Jong 

 time after their formation, either from remaining partly open, or 

 from the rapilli with which they are filled, leaving a kind of ditch, 

 which may be readily followed. They may be also recognised by 

 the partial and crate'riform excavations of these debris, which all 

 have the same line of direction ; sometimes they are distinguished 

 by rolls of scorias on the edges, which escaped while the lava was 

 boiling in the interior; they also exhibit conduits of lava, which 

 unite to each other the different cones of eruption formed on their 

 line of direction. It cannot be doubted that these cracks remain 

 partly filled with the lava to which they gave passage, giving rise 

 to veins, or dykes. Sometimes the lava flows above the crack or 

 fissure, forming sheets on the surface. Sometimes a coat or bed 

 of lava is found in evident communication with a dyke, which, 

 after having passed up through all the lower deposits, stops in thp 

 middle of it (^^. 201) ; and it is not rare to find several beds of 

 lava Jying one above the other, each one corresponding with a par- 

 ticular dyke (Jig. 202), to which, no doubt, it owes its origin ; the 



35. Are the characters of lava always the same? 



36. What are dykes? Are all dykes precisely the same in character 7 



