Ch. X.] 



PARALLEL SIDES OF DIKES. 123 



the mountain, and, whilst in a state of perfect fusion, continued 

 their course in those channels, which were sometimes full to 

 the brim, and at other times more or less so according to the 

 quantity of matter in motion. 



6 These channels, upon examination after an eruption, I 

 have found to be in general from two to five or six feet wide, 

 and seven or eight feet deep. They were often hid from the 

 sight by a quantity of scoriae that had formed a crust over 

 them, and the lava, having been conveyed in a covered way for 

 some yards, came out fresh again into an open channel. After 

 an eruption I have walked in some of those subterraneous or 

 covered galleries, which were exceedingly curious, the sides, 

 top, and bottom, being worn perfectly smooth and even in most 

 parts, by the violence of the currents of the red-hot lavas, 

 which they had conveyed for many weeks successively.' 



In another place, in the same memoir, he describes the liquid 

 and red-hot matter as being received ( into a regular channel, 

 raised upon a sort of wall of scoriae and cinders, almost perpen- 

 dicularly, of about the height of eight or ten feet, resembling 

 much an ancient aqueduct *.' 



Now, if the lava in these instances had not run out from the 

 covered channel, in consequence of the declivity whereon it was 

 placed if, instead of the space being left empty, the lava had 

 been retained within until it cooled and consolidated, it would 

 then have constituted a small dike with parallel sides. But the 

 walls of a vertical fissure through which lava has ascended in 

 its way to a volcanic vent, must have been exposed to the same 

 erosion as the four sides of the channels before adverted to. 

 The prolonged and uniform friction of the heavy fluid as it flows 

 upwards cannot fail to wear and smooth down the surfaces on 

 which it rubs, and the intense heat must melt all such masses 

 as project and obstruct the passage of the incandescent fluid. 



We do not mean to assert that the sides of fissures caused 

 by earthquakes are never smooth and parallel, but they are 

 usually uneven, and are often seen to have been so where volcanic 

 * Phil, Trans,, vol. Ixx. 1780. 



