904 



SCIENCE. 



[N. S. Vol. II. No. 52. 



are made up of combinations of these small 

 joint-planes and the cross fractures. (See Figs. 

 1 and 2.) 



Fig. 1. Feather fracture. The lines diverge to- 

 wards the margin of the divisional plane. 



Fig. 2. Ideal arrangement of planes and fractured 

 surfaces. A, principal joint-face ; e, e, margin of A ; 

 h, b, imbricated planes of fringe ; c, c, fractured sur- 

 face between b-planes ; b' b' and c' c', analogous to 

 bb and cc on A, giving rise to feather fracture. ' In 

 the b-plane on the right feather fracture is also 

 shown. 



The margin of joint-planes of this class fre- 

 quently dies out in a fringe in which these small 

 joints are much enlarged, the interval between 

 them is increased, and this is accomplished by 

 their being turned at a considerable angle, from 

 10 to 25 degrees, to the principal joint-plane. 

 These small joints also, where well developed, 

 show feather fracture lines diverging towards 

 the outer margin. Joint-planes are thus com- 

 plex surfaces of fracture. Over the surface of 

 the large joints the smaller joint-planes become 

 so closely set and so nearly parallel to the prin- 

 cipal surface of fracture that these smaller 

 fracture surfaces gradually disappear before the 

 unaided eye and become a mere granulation of 

 the joint surface. 



Where these joints are developed in a single 

 stratum, they are commonly in the Mystic 

 River argillite quarries in the form of elongated 

 elliptical planes, the main fracture dying out 

 above and below where the texture of the rock 

 changes parallel with the stratification plane. 

 The fringe of marginal joints then give rise to 

 a set of joints in the underlying and overlying 

 beds having a different direction from that of 



the main joint in the intermediate bed. Of less 

 frequent occurrence are discoidal joints, evi- 

 dently entire joint planes, of small size and cir- 

 cular in area because the stress which produced 

 them was relieved by a small fracture in es- 

 sentially homogeneous material. These vary 



Fig. 3. Discoidal joint, with b-planes and c-frac- 

 tures analogous to sj'Stem of fractures in fringe of el- 

 liptical joints; d,marginal conchoidal fracture area. 

 Arrows show dip of imbricated b-planes. 



from half an inch to three inches in diameter. 

 They consist of the small imbricated planes (b 

 in the diagram) and the cross ft-actures (c in the 

 figure). The author refrained at the present 

 stage of the investigation from expressing an 

 opinion as to the origin of these joints. The 

 subject was illustrated by typical specimens. 

 These joint structures also occur in the felsites 

 of Salem Harbor, the granitic and dike rocks 

 and in gneisses. A report on the investigation 

 is in preparation. T. A. Jaggae, Jr., 



Recording Secretary. 



NEW BOOKS. 



The Cambridge Natural SLstory, Vol. V. Peri- 

 patus, Adam Sedgwick; Myriapods, F. G. 

 Sinclair; Insects, DaIvid Sharp. London 

 and New York, Macmillan & Co. 1895. Pp. 

 xi -I- 584. $4.00. 



Text-book of the Embryology of Invertebrates. E. 

 KoESCHELT and K. Heider. Translated 

 from the German by Edward L. Mark and 

 W. McM. Woodworth. London and New 

 York, Macmillan & Co. 1895. Pp. xv + 484. 

 $4.00. 



Die Spiele der Thiere. Karl Geoos. Jena, 

 Gustav Fischer. 1896. Pp. xvi + 359. M. 6. 



Grundzuge der Marinen Tiergeographie. ARNOLD 

 E. Ortmann. Jena, Gustav Fischer. 1896. 

 Pp. iv + 96. M. 2.50. 



