718 ABSTRACTS OF PAPERS 



Professor Hobbs replied as follows : This criticism of Professor Davis is 

 already somewhat familiar, and of course is the first question which arises in 

 certain kinds of joint studies. The investigation which is here reported is 

 made along a line wholly neglected by the New England School of Geomor- 

 phology which Professor Davis has founded. Since, however, it is concerned 

 with the lines of special excavation by eroding agencies, its importance is of 

 the first rank in geomorphologic studies. 



Prof. J. F. Kemp: I am convinced that there is much truth in the feature 

 as presented by Professor Hobbs. All of us who have worked in the Adiron- 

 dacks have been impressed with the marked system of northeast and north- 

 west precipitous escarpments and valleys, and in the eastern mountains with 

 an older north and south and east and west series whose sides show more 

 protracted erosion. The map used by Professor Hobbs from the southwestern 

 corner of the Elizabethtown sheet is a striking illustration. The drainage 

 relations were first emphasized by Professor Brigham, who called the system 

 "trellised drainage." We have since shown the dependence of the streams 

 upon faults. 



APPARENT SUN-CRACK STRUCTURE IN DIABASE 

 BY EDGAR T. WHERRY^ 



(Abstract) 



The upper surface of the great diabase sill of the Newark Group, in Mont- 

 gomery County, Pennsylvania, shows at several points a network of lines 

 closely simulating sun-cracks. Thin-sections show these lines to consist of 

 streaks of coarsely crystallized augite and feldspar traversing the fine grained 

 groundmass. They are probably to be regarded as shrinkage cracks devel- 

 oped by the sudden cooling of the molten diabase against the shale surface, 

 filled up by molten material from the interior of the mass. Boulders of 

 weathering have also been observed showing hexagonally arranged cracks, but 

 these are entirely of secondary origin, no structural peculiarity being observa- 

 ble in connection with them. 



Discussion 



Dr. G. W. Stose : This same phenomenon has been seen on the upper con- 

 tact surfaces of diabase sheets in southern Pennsylvania, and is regarded as 

 produced by shrinkage on rapid cooling and incipient formation of polygonal 

 blocks. 



GEOLOGY OF PART OF LUNA COUNTY, NEW MEXICO 

 BY N. H. DABTON 



{Abstract) 



During the past autumn an examination was made of the structure of the 

 Cooks Peak Range, Florida Mountains, and some adjoining ridges in south- 

 western New Mexico. They present an extensive sedimentary succession and 

 a series of varied igneous rocks. The structural features throw considerable 

 light on the character of desert ranges. The great bolsons, separating the 



1 Introduced by Benjamin L. Miller. 



