84 



SGIENGE. 



[N. S. Vol. V. No. 107. 



This paper consisted of a study of the 

 various conditions affecting erosion for the 

 purpose of determining the origin of cer- 

 tain exceptional surface features which have 

 not hitherto been satisfactorily explained. 

 The writer reaches the conclusion that local 

 radial movements of the crust of the earth 

 are largely responsible for the exceptional 

 physiographic features. 



This theory was then applied to the N"orth 

 Carolina section of the Blue Eidge, and it 

 was shown that the striking features of that 

 region can be explained by supposing a 

 monoclinal uplift to have occurred along 

 the eastern front of the Eidge while the 

 main body of the Piedmont plain remained 

 at sea level. On this assumption the various 

 remnants of peneplains in the Blue Eidge 

 region correspond to halts in the general 

 uplift, and they collectively represent the 

 same, or a portion of the same time inter- 

 val which is marked by the Piedmont plain 

 to the eastward of the uplift. 



Since local uplifts are liable to occur at 

 any time and in any place, the resulting 

 forms may be found in other localities than 

 the Appalachians, and physiographers 

 should learn to distinguish them, for they 

 constitute a record of exceptional condi- 

 tions. 



The paper was illustrated by a large map 

 which brought out the position of the Ap- 

 palachians south of Eoanoke, Va., to which 

 the hypothesis especially applied, but the 

 region of the Black Hills was also cited as 

 a promising place in which to test it. 



Mr. Keith, in discussion, spoke in opposi- 

 tion to Mr. Campbell's view and sketched 

 a district on the Nahantahala and Hiwassee 

 rivers in support of his contention. It was 

 later shown that the district, however, was 

 not in the region described by Mr. Campbell 

 and had but limited bearings on his thesis. 



Homology of Joints and Artificial Fractures. 

 J. B. WooDwoETH, Cambridge, Mass. 



A synopsis of the structure of the typical 

 joint was given from a previous paper (Proc. 

 Boston Soc. Nat. Hist., Vol. XXXIV, 1896, 

 pp. 163-183). A series of specimens show- 

 ing the relations between joints and artifi- 

 cially fractured surfaces was exhibited. Ob- 

 liquity of the biplanes in the margin of frac- 

 tures is found of value in determining the 

 axis of breaking tension arising from ten- 

 sion, contraction or shearing force. Inci- 

 dentally, a map symbol for joints was pro- 

 posed. 



The paper was illustrated by an excellent 

 and striking series of lantern slides which 

 made the matter clear. 



Physiographic Development of the District of 

 Columbia Region. N. H. Daeton, Wash- 

 ington, D. C. 



An outline of the physiographic condi- 

 tions which have characterized each of the 

 earlier stages in the Coastal Plain history 

 of the region and a description of the de- 

 velopment of the present topography was 

 presented in a very interesting manner. 

 By means of lantern slides from maps and 

 photographs, the ups and downs of the old 

 shore-lines were shown and the formation 

 of the terraces. In this way the develop- 

 ment of the Potomac, Magothy, Severn, 

 Chesapeake, Lafayette and lower and upper 

 Columbia formations were traced. 



Dikes in Appalachian Virginia. N. H. Dae- 

 ton, Washington, D. C. 

 An announcement of further discoveries 

 of diabase dikes and of interesting dikes of 

 acidic eruptives among the Paleozoic rocks 

 of central Appalachian Virginia. When 

 the specimens were passed around, the acidic 

 intrusions, containing, as they did, quartz, 

 feldspar, mica and hornblende pbenocrysts 

 in a white ground mass, excited extreme 

 interest, and, as remarked by J. E. Wolff, 

 were very similar to the quartz-mica-por- 

 phyrite in the Cortlandt series, near Mont- 

 rose, N". Y. 



