PROCEEDINGS 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. VII, pp. 267-275. Plate XII. July 24, 1905. 



SIMULTANEOUS JOINTS. 

 By George F. Becker. 



Joints are almost universally distributed over rock expo- 

 sures, and they are so highly significant that the interest attach- 

 ing to them can never be exhausted. In the present paper I 

 propose to discuss systems of joints of simultaneous or almost 

 simultaneous origin, not with the idea of developing any new 

 principles, but in order to call the attention of geologists and 



mining men to some details which have been insufficiently con- 

 es ^ 



sidered although they are of importance in reading the record 

 of mining districts and tectonic belts. 



Most fine-grained solids which are capable of rupture under 

 given conditions behave similarly. Exceptionally plastic or duc- 

 tile bodies, like aluminium and pure lead, can scarcely be broken 

 by crushing. Some substances again show different resist- 

 ances in different directions ; for example, single crystals, like 

 those of quartz, and masses with a laminar structure, such as 

 slate. But massive rocks in large masses, as well as many 

 limestones and sandstones, cast iron and some forms of steel, 

 are to all intents and purposes isomorphous in that they display 

 practically equal resistances in all directions. Such materials 

 when subjected to forces obey the same laws as softer solids, 

 such as plaster of paris, wax and " ceresin " (the trade name 

 for a mixture of crystalline parafiines derived from ozokerite). 

 It would indeed be perplexing if large blocks of materials com- 

 posed of small crystalline grains irregularly oriented, did not 

 show common properties.' Even clay, so little moistened as to 

 be " stiff " acts as if it were a true solid. 



'With glasses, a class of bodies which needs more study than it has received, 

 I shall not deal in this paper. 



Proc. Wash. Acad. Set., July, 1905. 



267 



