FAN STRUCTURE 319 



that their formation can not be due wholly to the ordinary processes seen 

 in mountains caused by tangential shortening ; the depression of the geo- 

 synclinal area and the uplift of the corresponding geanticline, of course, 

 actually took place. The amount of departure of the beds from the hori- 

 zontal is very much less, however, than in the folds due to lateral com- 

 pression, and the kind of uplift is precisely that seen in the gentle 

 doming of the Cretaceous and Tertiary beds around the Appalachians 

 and the similar, though still slighter, doming of the later peneplains 

 throughout the Appalachians. In these there is no evidence whatever of 

 lateral compression, and a cause for them must be sought in which ver- 

 tical movements or oscillations are the important feature. With this 

 point in view it would perhaps be better to separate anticlines and syn- 

 clines, due clearly to horizontal compression, from the low domes and 

 basins which form geosynclines and geanticlines and which are, in the 

 main, due to vertical movements. 



FAN STRUCTURE 



In many areas east of the Blue Eidge uplift variations are seen from 

 the rule of Appalachian structures. In practically all Appalachian folds 

 the axial planes are either upright or overturned toward the northwest, 

 but folds overturned toward the southeast are found in a fairly well 

 defined belt from Alabama to Massachusetts. Evidence of this is rather 

 scarce and usually is obtainable only where sediments are involved with 

 the gneisses and granites; it consists of folds closed and overturned to 

 the southeast and of thrust faults dipping to the northwest. Near Wash- 

 ington secondary structures in gneisses exhibit the same features. These 

 structures, taken in connection with the usual southeastward dips, form 

 a great fan structure. 



The fan structure is well defined at the following distances from the 

 structural line of the border of the Appalachian Valley : Alabama-Geor- 

 gia boundary, 75 miles; South Carolina-North Carolina boundary, 95 

 miles ; North Carolina- Virginia boundary, 40 miles ; Maryland, 40 miles ; 

 and Massachusetts, 90 miles. A second belt parallel to the above and 

 about 50 miles to the southeast is seen in North Carolina, where the 

 Appalachians are widest. Between Maryland and Ehode Island these 

 structures are covered by the Cretaceous sediments or the Atlantic waters, 

 and they reach the ocean again in the Boston basin. Their position in 

 central Maine is not well known, for lack of detailed work, but they lie 

 near the mouth of Penobscot River. 



The belt of fan structures thus defined has an important relation to 

 the large groups of granite batholiths. Where a considerable area is ex- 



