60 PROrEEDINGS OF THE ANN ARBOK MEETING 



taiice e<iual to 20 times the diameter of the perfect central fold. Parallel 

 folds represent 2:reat shortening: along the central axis, which dies out to 

 nothinir above and below that axis. It represents a rotational strain with 

 maximum displacement along a plane. The inverse of this fold would be rep- 

 resented by zero shortening along the central plane, increasing to a maximum 

 shortening in both directions from that plane. In this case the folds would 

 not die out from the central axis, but they would increase with distance from 

 the axis. These features are extremely rare, but are represented by the so- 

 called boudinage of southeastern Belgium. The origin of rotational strains 

 which are competent to make boudinage is obscure. Certainly it has no t\in- 

 nection with normal folding. Tt is suggested that it represents a hitherto 

 unrecognized type of mnjor structure caused by longitudinal compression re- 

 lieved by central bulging. 



Presented from notes. 



With this paper the morning session closed at 1*2.40 o\loc-k. 



GHorr rnoTOGRAPH 



At the close of the morniiiii- sessions those of the memhers in attend- 

 ance who could be assembled on the steps of one of the university build- 

 ing's had a group photograph taken. 



TXXCTTKOX GIVEN liY THE UNIVtlRSlTY 



At 1 o'clock members of the Society, together witli those of affiliated 

 and associated organizations, were entertained at luncheon by the univor- 

 sitv in the ^lichia'an Union. 



Session's of Friday Afterxoox 



Two sectional meetings, to be held simultaneously, were organized for 

 Friday afternoon. The papers of Group A — dynamical, structural, gla- 

 cial, and physiogra]ihic subjects — were presided over by Vice-President 

 Wa^ihington as iliairman. and Vice-President Robert T. Hill was chair- 

 man of the section in whicli the papers of Grouj) B. on stratigrajihic. 

 })aleontologic, areal, and cartographic subjects, were presented. 



There wa^ also a joint session with the Society of Economic Geologists 

 for the discussion of the ore deposits of the copper-bearing rocks of 

 Michigan. 



SECTIONAL MEETIN(t OF FHIDAY' AFTERXOON FOR GROFR A 



The section for the reading of papers on dynamical, structural, glacial, 

 and physiographic subjects met at about *2.oO o'clock, in the Auditorium, 



