422 W. M. DAVIS AND S. W. LOPER — FOSSILIFEROUS TRIASSIC SHALE. 



microscope, minute fragments of trap are seen to be mixed with the sand 

 grains in the filling of the vesicles. Being extrusive, the lava sheets may 

 be regarded as conformable members of the bedded series ; being resistant, 

 they form ridges and are easily traced ; hence much of the stratigraphic 

 study of the region is based upon them. In the second place, when the 

 region is examined on several northeasterly lines parallel to one another, the 

 sequence of beds on each line is found to be essentially constant, namely, 

 lower sandstones and conglomerates, a vesicular lava sheet (the anterior), 

 shales, a heavy lava sheet (the main), shales, a thin lava sheet (the posterior), 

 and finally an upper series of shales and sandstones. This repetition of so 

 complicated a sequence of beds is not thought to be possible as an accidental 

 occurrence ; hence the hypothesis of faulting is introduced, and the fault 

 lines are searched out. These, in the third place, are found by tracing the 

 ridge-making members of the series until they end, and drawing Hues to con- 

 nect their terminations. The lines thus drawn are found to run systematic- 

 ally northeast-and-southwest ; bands of breccia are found at several points 

 along them ; local disturbances of the generally uniform dip of the beds 

 occur along the lines, and always acoordant with the drag of the supposed 

 faults; the heave of the faults is, with one exception, systematically on the 

 eastern slide of the line of fracture. In the fourth place, the peculiarly 

 intricate relations of the ridges, by which they are offset from one another 

 and their ends overlapped — the "advancing" and "receding order" of Per- 

 cival, — find a simple geometrical explanation, susceptible of trigonometri- 

 cal formulation, by means of the theory of faulting; and the prevalence of 

 abrupt bluffs at the southern ends of the ridges, while the northern ends fall 

 away gradually (a very marked feature of the Meriden district) follows 

 necessarily from the degradation of monoclinal lava sheets cut by oblique 

 faults, the bluffs being formed where the strike of the sheets and the trend 

 of the faults make an acute angle. 



When evidence so varied and so complete is repeated over and over again 

 in the most systematic order, the conclusion to which it leads cannot be gain- 

 said. 



But even though the faults are seemingly demonstrated fully by general 

 structural evidence, additional evidence is always in order; and for that 

 reason I have endeavored during the past summer to discover whether the 

 fossils of the formation would bear on the question. 



If a fossil-bearing horizon is found in one of the blocks into which the 

 formation is divided, it evidently might be expected to occur in the same 

 position relative to the trap sheets in the adjoining blocks, and so on for a 

 considerable distance from the place of its original discovery, as indicated in 

 figure 3. A correct knowledge of the location and throw of the various faults 

 should thus enable one to define with considerable accuracy the localities Avhere 

 outcrops of any fossiliferous bed might be looked for in the several blocks. 



