THE VOLCANOES OF THE TRIASSIC PERIOD. 



417 



The original extent of the complete formation beyond its present bound- 

 aries is not known. It may have been many miles broader than it now is, 

 but there is no good evidence now in hand on this question. 



Formation of the Trap Sheets. — During the period of deposition there were 

 outflows of lava at several dates. The sheets formed by outbursts at three of 

 these dates are now well correlated ; the first of them is relatively thin and 

 vesicular or amygdaloidal ; the second is much thicker and more massive, 

 and at some places it appears as a double flow, one sheet of lava lying on 

 another without a noticeable accumulation of sediments between them ; the 

 third is again thin like the first. The outcropping edge of the massive 

 middle sheet now forms a series of strong ridges ; hence it was called the 

 " main " trap sheet by Percival, while the lower and upper were called the 



Figure 1 — A Portion of the completed Triassic Formation lying on the denuded Crystallines. 



The lower lava bed, spreading out from the dikes near the bottom of the Triassic strata, is the 

 intrusive sheet. The three upper sheets, spreading out from the volcanic cones, are the overflows, 

 called respectively the anterior, main and posterior. 



" anterior " and " posterior " sheets respectively, although he did not recog- 

 nize that these names, which he used to indicate relative topographic posi- 

 tions, would indicate relative time of outflow as well. 



In addition to these great overflows, there is at least one great intrusive 

 sheet* and, apparently, several smaller ones. The large one occurs close to 

 the base of the formation, and hence is no^v seen near its western margin, 

 because the present structure is, as a whole, an eastward-dipping monocline. 

 The date of this intrusion is not definitely fixed, although it appears that 



* Professor Newberry has misquoted me (Monograph XIV, U. S. Geol. Survev, 1888, p. 7) as saying 

 that all the trap sheets were overflows. The intrusive nature of the West rock and palisade sheets 

 was recognized in my first essay, and I have never found reason to regard it as of other origin. 



