332 A. KEITH OUTLINES OF APPALACHIAN STRUCTURE 



England and Canada in the late Devonian and early Carboniferous, bnt 

 it was muck weaker and limited in area. 



Between these two major kinds of deformation there are man}' of 

 intermediate nature. A few of the vertical movements were concentrated 

 here and there enough to produce actual and visible folds. Such move- 

 ments took place in the Lower Ordovician of Tennessee, Georgia, and 

 Alabama, and at a similar period in Pennsylvania, New York, Vermont, 

 and eastern Canada. Between the Ordovician and Silurian a slight 

 movement of this nature took place in a small part of southeastern New 

 York and in southeastern Maine. This has been called the Taconic 

 revolution, but it is too unimportant for such a designation, and it is 

 not exhibited in the Taconic Range. In these there was a small element 

 of horizontal movement. In many of the other vertical movements it is 

 difficult to determine whether or not the crust moved laterally. In addi- 

 tion to general upward movements there were those of tilting, during 

 which erosion quickened and the seas and their sediments expanded enor- 

 mously. A major movement of this kind took place late in the Cam- 

 brian, spread Paleozoic sediments for the first time over the interior of 

 the continent, and raised Appalachia east of the interior sea. This was 

 the real beginning of the Appalachian system. All of these kinds of 

 deformation have acted on the Appalachian region and have united in a 

 record of great complexity. 



The Appalachian region has been affected by strong deformation since 

 the Appalachian revolution. Mention has already been made of the 

 Triassic dislocations and their general accordance with those of the 

 Appalachian system. As has been stated, the trends of the Triassic 

 structures exhibit just such broad curves as are so plain in the Appa- 

 lachian folds. The Triassic structures, however, strike somewhat more 

 nearly north and south than the Appalachian structure, in common Avith 

 similar trends in the Precambrian structures. In Xorth Carolina the 

 Triassic structures are only seen in the eastern part of the Appalachian 

 area where it is broadest. Passing northward through Virginia the 

 Triassic basins transgress slowly westward on the Appalachian folds until 

 in southeastern Pennsylvania they have crossed the Blue Ridge axis into 

 the Great Valley. Thence northeastward they recede somewhat from 

 the valley belt of folded rocks, but continue in or just east of it in the 

 Hudson and Champlain valleys of New York. East of these valleys the 

 Triassic structures are well developed in Connecticut and Massachusetts 

 and continue up the Connecticut Valley in Vermont. 



The broad structural units of that time consist of three great blocks of 

 the crust, the central one 80 or more miles wide from east to west. This 



