516 DAVIS AND GRISWOLD — BOUNDARY OP CONNECTICUT TRIASSIC. 



ture of the formation within its boundaries, which were examined only 

 incidentally until especially studied in recent summers by the junior au- 

 thor. This paper is chiefly concerned with the results thus determined 

 along the eastern boundary of the formation. Our problem is in effect : 

 How closel}^ does the present border of the Triassic formation follow its 

 original constructional border, and how much has it been modified by 

 deformation and erosion? This problem must be entered with a clear 

 understanding of the results already gained regarding the structure of 

 the formation as a whole, which are therefore briefly reviewed here. 



Geological History of the Region. 



The Triassic conglomerates, sandstones and shales lie on a complex 

 foundation of cr3^stalline and metamorphic rocks which were greatly 

 eroded before Triassic time. The unconformable relation of the two is 

 wonderfully well displayed on the western border of the formation in a 

 ravine west of Southington, some twenty miles north of New Haven. 

 A belt of the old crystalline land area seems to have been depressed and 

 submerged, and this belt as it sank continually received the waste from 

 the more elevated parts. Thousands of feet of sediments were thus»ac- 

 cumulated, the surfaces of successive strata having been frequently ex- 

 posed to shallow currents, as is shown in ripple-marks, to sunshine, as 

 testified to in mud-cracks, and to passing showers, as indicated by rain- 

 drop imprints. Fish swam in the clearer waters, where fine shales were 

 formed ; reptiles walked over the mud-flats, marking their paths by the 

 so-called " bird-tracks," for which the valley is famous. The sediments 

 were coarser near the eastern and w^estern margins, where wave and 

 current work were active, occasionally supplying bowlders up to several 

 feet in diameter, while the contemporaneous deposits in the more open 

 waters of the middle area w^ere generally finer. We do not find any 

 facts whose explanation requires the aid of glacial action. Vast volcanic 

 eruptions interrupted the more quiet processes of sedimentary deposition 

 by the intrusion of dikes and necks of diabasic lava, by the discharge of 

 outbursting ashes, by the outpouring of broad lava flows, and by the in- 

 trusion of deep seated lava sheets. These lavas will be here referred to 

 by the popular term, trap. 



The materials accumulated maybe summarized as follows : Resting 

 on the crystalline foundation there are generally from 200 to 1,500 feet of 

 bottom sandstones and conglomerates, beneath an intrusive sheet of trap 

 about 650 feet thick ; then follow from 5,000 to 6,500 feet of lower sand- 

 stones and conglomerates ; 250 feet of an amygdaloidal trap sheet, some- 

 times with ashes and lava blocks, indicative of explosive eruption and 

 called the anterior trap sheet; 300 to 1,000 feet of anterior shales and 

 sandstones, including a black shale bearing fossil fish: 500 feet of heavy 



