PRODUCTION OF VOLCANIC DYKES. 



479 



eastern outlet, and descended by the Canale del Inferno. Of the whole 

 of the numbered dykes, ten present as their main feature an approxi- 

 mately vertical plane, all the rest having a very sensible dip or hade. 

 This dip, from No. 1 to No. 13, is towards the west and north at 

 various angles, none exceeding 38 degrees with the vertical. Be- 

 tween Nos. 13 and 27 the dip trends towards the east and north, never 

 exceeding from 30 to 40 degrees, and the average being not more 

 than about 20°. 



I did not observe any case in which a dyke was intersected by a 

 coherent bed of lava ; but in one instance the highest part of one 

 dyke (No. 1) is stopped at its upper extremity by the underside of 

 an overlying and nearly horizontal bed of lava. Not only do many 

 dykes bifurcate or send off branch dykes at different heights, but two 

 dykes frequently intersect each other and often at considerable 

 angles, so that in one instance (No. 10) the general plane of one is 

 nearly orthogonal to that of the other. 



These and many other circumstances prove these dykes to have 

 been produced at very different and successive ages. Many of the 

 curved, twisted, or broken dykes present proofs of having been sub- 

 jected to severe and long-continued pressures in both vertical and 

 horizontal directions, which have resulted in great displacements 

 from their original positions. Thus the third dyke examined, between 

 No. 13 and No. 14, has been crushed by descending movements of 

 the surrounding matrix, as roughly indicated in fig. 1. In others 

 lateral displacement has attended also upon vertical movement, as in 

 the dyke No. 9, which has been cross-fractured with lateral displace- 

 ment and descent of one segment of the dyke, as indicated in fig. 2. 

 An ideal figure of this form of displacement has been given by 



Fig. 1. — Dyke crushed 

 by descending move- 

 ment of matrix. 



saw* ~&**v* 



Fig. 2.— Dyke frac- 

 tured with lateral 

 displacement. 



^J^kL 



Fig. 3. — Curved 

 Dyke. 



v^J^S 



Phillips, 'Vesuvius/ page 132, whose very slight notice of these 

 dykes in that work leaves much to be desired. A third form of dis- 

 placement is seen in the first dykes examined, between No. 22 and 

 Q. J. G. S. No. 128. 2 l 



