PRODUCTION OF VOLCANIC DYKES. 487 



adjacent, while the opposite side of the dyke has its irregularities 

 mainly convex to the general surface, and sometimes running into 

 wrinklings such as do not occur on the first-mentioned side. 



This difference always characterizes and distinguishes the bottom 

 from the top surface of lava beds which have flowed over slight 

 slopes, and which have not become superficially broken up into lumps 

 of scoria : the lower surface takes concave markings from protuberant 

 or irregular portions of the surface over which it has run, and 

 contains stones and pebbles imbedded in the mass which have been 

 picked up from that same surface ; the upper surface of the lava 

 presents, on the contrary, irregularities which are in the main 

 convex, protuberant, or bombees. 



This diverse character at opposite sides of the same dyke, how- 

 ever, is obscurely presented by some of the dykes examined by 

 me, in which pebbles are found imbedded in one side and none in 

 the other, although the matrix at both sides appeared the same. In 

 others one side only shows the superficial change into vitreous 

 matter, while the other shows no such change, — the more general 

 fact, however, being that both surfaces of the dyke show a vitreous 

 crust, which is often but a fraction of an inch in thickness. The 

 following dykes show in a marked manner these disparities of the 

 two opposite surfaces : — In No. 6 A, 31 feet thick, a thin vitreous crust 

 at the south-west side, at the opposite side a thick vitreous crust, 

 with pebbles imbedded in the surface. In dyke No. 8, 8 feet thick, 

 there are large imbedded but not adherent pebbles on the east face, 

 none on the other, which is covered with a wrinkled crust. In dyke 

 No. 9, thickness 3| feet, large and small imbedded pebbles on the 

 north-east side, with adherent blackish scoriae; opposite side no im- 

 bedded or adherent matter, and irregularities of the surface convex or 

 bombees. Dyke No. 20, 6 to 7 feet thick, many imbedded pebbles, 

 some as large as a man's head in east face ; opposite sides wrinkly 

 and scoriaceous without pebbles. Dyke No. 22, 3 to 3| feet thick, 

 east side hollow, with some pebbles and cavities, and wrinkled ; west 

 side bombe, and contains many air-bubbles. Dyke No. 24, 8 to 9 

 feet thick, north side bombe irregularly, no crust, pebbles, or 

 cavities ; south side with deep large and small pits or cavities, and 

 with adherent crust and pebbles. It will be noticed that these dif- 

 ferences in opposite surfaces are most observable where the dykes 

 themselves are thick, indeed are not seen at all in the thin dykes. 

 The vitreous external crusts generally present the mineral cha- 

 racters of obsidian passing into a less vitreous material, having the 

 external character of basanite or Lydian stone, and always very 

 dark-coloured, the fracture vitreous and subconchoidal. I can only 

 vouch for the above particulars as affecting such a height in each 

 dyke above the floor of the Atrio as I was enabled to climb to and 

 examine. It is remarkable that no pyromorphic change or action 

 is noticeable in the material lying in contact with or adjacent to any 

 of these dykes. The disparity in the characters of opposite faces 

 here described appears to be very difficult of explanation, and 

 presents a proper subject for further observation here and in other 



