MUSEUM OF COMPArwVTIVE ZOOLOGY. 289 



almost wholly confined to the lower portion of the volcanic series" 

 (295). 



A. Geikie. On the Carboniferous Volcanic Rocks of the Basin of the 

 Firth of Forth. Edinb. Roy. Soc. Trans., XXIX., 1879, 437-518. 

 Both intrusions and overflows are again recognized ; in the former, a 

 cellular or amygdaloidal texture is hardly to be observed, and never 

 when they are largely crystalline (475) ; in the latter, amygdaloids are 

 common (481). 



If the trap sheets are old lava-overflows, there must of course be 

 supply dikes, by which the sheets were fed, and these dikes must cut 

 across and bake and be younger than all the strata they pass through, 

 but their cross-section will probably be small compared to the area 

 of the overflows ; the sheets will be rather closely conformable to 

 the strata over which they flow, though, as miglit be expected, their 

 creeping advance while yet molten may have produced some disturb- 

 ance, and they may contain sandstone fragments ; only the previously 

 formed sandstone beneath them can be baked ; the upper and lower 

 surface of the flow should differ as in modern lava flows, the upper being 

 more vesicular and uneven than the lower ; one flow may cover another ; 

 the compact lavas may be preceded or followed by tufaceous or frag- 

 mentary deposits ; the overlying sandstone must lie conformably on the 

 uneven surface of the lava, adapting itself to all inequalities, and grad- 

 ually filling them to an even surface ; it may often contain volcanic sand 

 or fragments of lava. 



Overflow sheets are much more common than intrusions ; the follow- 

 ing references will lead to descriptions of some of the more notable. 



K. C. V. Leonhard. Die Basalt-Gebilde, 1832. This work serves 

 well as a guide to the older European observations on eruptive rocks. 

 Both overflow and intruded sheets are recognized ; the former are de- 

 scribed as scoriaceous on the surface, while the latter are generally dense, 

 because gas bubbles could not expand in their heavily compressed mass 

 (I. 473) ] but in speaking of the baking in the adjoining strata (II. 230), 

 the author does not clearly state the different effects of the two kinds 

 of sheets. 



J. W. Dawson. On the Lower Carboniferous Rocks, or Gypsiferous 

 Formation of Nova Scotia. Geol. Soc. Journ., I., 1845, (29-31). Sev- 

 eral beds of trap conformably interbedded with the adjoining strata; the 

 trap is dense below, amygdaloidal above, and its fragments are found in 

 an overlying conglomerate. (See also e, 316, and section, 125; /t, 49.) 



J. S. Newberry. Pacific Railroad Reports, Vol. VI., 1857, ch. vi., vii., 



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