NO. 23.] CENTRAL CONNECTICUT IN THE GEOLOGIC PAST. 19 



ancient sea terraces become of importance. Little suggestion of 

 them, however, can be seen by a casual study of the landscape. 

 It is rather a comprehensive study of topographic maps which 

 supplies the evidence, but a final conclusion on this subject must 

 await a detailed publication. 



The Rock Structure. The erosion surface gives the data for 

 deciphering one side of geologic history, that of the surface 

 activities; the rock structure gives another side of this history, 

 that connected with the forming and transforming of the rocks. 



The structure section shown in Figure I shows the attitude 

 and nature of the rock formations, the oldest being united in one 

 group the pre- Paleozoic complex gneisses. Back of the Paleo- 

 zoic ages lies a tangled record, which speaks, however, of eras 

 of mountain-making, erosion, and sedimentation, followed at last 

 by a manifestation of mountain-making forces on a prodigious 

 scale. The sediments were crystallized, mashed, and injected 

 with sheets and masses of molten rock, thus developing the pre- 

 Paleozoic complex gneisses, the result of internal forces so vast 

 as to remake the crust and everywhere hide in obscurity the earli- 

 est history of the earth. This " Basement Complex " does not 

 rise to the surface on the line of the structure section, its nearest 

 outcrops being in the northwestern portion of the State. 



The second group of rocks shown in the drawings comprises 

 the Paleozoic sediments. During the greater part of that era 

 most of the area of Connecticut was, as now, a part of the land, 

 but then, in marked contrast with present conditions, it stood on 

 the eastern side of an inland sea. Long Island Sound was not yet 

 in existence, and the Appalachian continent, now in large part sub- 

 merged, extended to the south. The mountain system was fur- 

 thermore subjected more than once to movements of folding and 

 uplift. The Paleozoic sediments therefofe represent only certain 

 periods when the land stood lowest and the sea held widest sway. 

 But not all of the sediments are positively marine, some of them 

 may have been formed as delta deposits skirting uplands and 

 built out against a western shallow sea. Only portions of the 

 Paleozoic sediments have been preserved, that is, the parts which 

 were folded down rather than thrust up. The folding, mashing, 

 and crystallization to which these sediments were subjected in 

 mountain-making movements of the Paleozoic, especially near 



