288 



BULLETIN OF THE 



Heinrich accepts the general view of post-Triassic intrusion for the 

 Virginia traps (251). Fontaine writes that the " outpour " of fused rock 

 occurred near the end of the period of deposit (29); but his attention is 

 given more especially to the stratified rocks. Farther south than Vir- 

 ginia, there is no mention of anything but dikes in the sandstone. 



It would thus seem that the overflow theory of the origin of the 

 trap sheets has found its most pronounced supporters north of Con- 

 necticut. 



There is theoretically no difficulty in distinguishing between the in- 

 trusive and overflow modes of origin, and the practical difficulty and 

 difference of opinion above shown are probably to be explained by the 

 rarity of good points of observation. 



If the traps are intrusive, the sandstone above and below should show 

 about equal signs of metamorphism ; there might be fragments of sand- 

 stone in the trap, and such should be well baked, but there could be no 

 fragments of trap in the sandstone; the trap might break across the 

 sandstone layers, or send branches off from its main body ; the upper 

 surface of the trap should not be scoriaceous, especially in the thin 

 layers, for if intrusive it must have been under almost as much pressure 

 as the lower surface. 



Examples of intrusive sheets elsewhere are described by the following 

 authors : — ■ 



G. K. Gilbert. Geology of the Henry Mountains (Utah), 1877. The 

 intrusive rock is not vesicular at all ; no fragments from it occur in 

 the overlying strata ; metamorphism is marked as well above as below 

 the intrusions. The name of laccolite was proposed by Gilbert for such 

 masses of eruptive rock, and it may be applied to the Triassic intrusions 

 as well. The laccolites of the West still retain their original horizontal 

 position : those of the East have been tilted since intrusion. 



Other less detailed mentions of intruded sheets may be found in the 

 Annual Reports of the Geological Survey of the Territories, 1873, 18G 

 (Marvine), 234(Pea]c); 1874, 04 (Holmes), 219 (Endlich); 1875, 60, 

 95 (Pealc), 268 (Holmes); 1876, 194 (Holmes): in the Bulletin of the 

 same Survey, III. 1877, 551-564 (Peale). 



A. Geikic. On the Tertiary Volcanic Eocks of the British Isles. 

 GcoL Soc. Journ., XXVIL, 1871, 270-310. The flows and intrusions of 

 the island of Eigg are described in detail : the overflows have frequently 

 been regarded as intrusions (292), but the two are easily separated 

 by the difference in their metamorphic effects, and by the presence or 

 absence of slaggy, amygdaloidal structure (281); the intrusions "are 



