206 HISTORICAL GEOLOGY 



Thickness of the System and Igneous Rocks. — Figures show- 

 ing the thickness of the system in different parts of the continent 

 have already been given. Igneous rocks are abundant only in 

 British Columbia, Alaska, and also in the Newark series of the 

 Atlantic Coast, the last named being again referred to below. 



Physical History 



Atlantic Coast. — Accompanying map Fig. 123 shows the geog- 

 raphy of the continent during Triassic time, though it must be 

 borne in mind that the areas of deposition along the Atlantic Coast 

 were such only in the latter part of the period. 



The non-marine strata (Newark series) of Upper Triassic age 

 clearly show by their present distribution and mode of occurrence 

 that they were deposited in a series of long, trough-like depressions 

 whose trend was parallel to that of the main axis of the Appalachian 

 range. These troughs lay between the Appalachians proper and 

 old Appalachia, which latter was then also, in part, made up of the 

 greatly worn down Taconic Mountains. The fact that these 

 troughs are true downwarps, and that they so perfectly follow the 

 trend of the Appalachian folds, makes it certain that they were 

 formed by a great lateral pressure which was a continuation of the 

 Appalachian disturbance. Thus the Appalachian Mountains still 

 seem to have been growing well into the Triassic period and, while 

 the Paleozoic strata were being folded, the surface of old Appa- 

 lachia, including part of the Taconic region, was also more or less 

 warped, the downwarps forming the troughs in which the Triassic 

 beds were deposited. These depressions or troughs were most 

 favorably situated for rapid accumulation of thick deposits be- 

 cause of their position immediately between the two great land 

 masses which were being eroded. The sediments derived from 

 the erosion of the young Appalachians were especially abundant 

 because of the vigorous wearing down of these newly formed 

 mountains. Thus the great thickness of the Newark series is 

 accounted for. Their thickness strongly argues for a gradual 

 downwarping of the basins as the deposition of sediments went on. 

 It is often stated that these strata were formed in estuaries, but the 

 presence of sun-cracks, ripple-marks, Reptile tracks, etc., show 

 that, in part at least, the beds may have formed in very shallow 



