1885.1 243 [Crosby. 



He also showed a sketch of a wing of a middle silurian insect, from 

 France, which Brongniart thought to be a blattarian, but which 

 seemed to the speaker rather a neuropteroid. Another recent dis- 

 covery is that of several Coleoptera from the " Kulm-schichten" 

 of Silesia, the oldest known members of this order. 



The President then introduced Mr. Frank H. Cushing, who gave 

 a most interesting account of a Zuni's theories of many natural 

 phenomena and of his ideas about the origin of various animals and 

 plants. 



General Meeting, Dec. 16, 1885. 



The President, Mr. S. H. Scudder in the chair. 

 The following paper was read : 



Notes on Joint Structure. By W. O. Crosby. 



In a communication 1 presented to this Society three years ago, 

 I proposed to recognize three classes of joints, as follows : (1) the 

 parallel and intersecting joints which are especially characteristic 

 of stratified rocks ; (2) the non-parallel and non-intersecting joints 

 observed chiefly in eruptive rocks and having their best develop- 

 ment in the columnar jointing of the basaltic rocks ; and (3) the 

 comparatively unimportant joints parallel with the surface so 

 clearly exposed in many granite quarries, and rarely observed ex- 

 cept in very massive, granitoid rocks. 



This third class of joints has been explained by Prof. Shaler, 

 apparently to the general satisfaction of geologists, as due to ex- 

 pansion caused by the sun's heat from day to day and from season 

 to season. Hence, in allusion to their origin, these may be very 

 properly designated as expansion joints. The second class, after 

 having been for many years variously explained as a species of 

 crystalline or concretionary structure, is now, by the common 

 consent of geologists, regarded as originating in the contraction 

 of the rocks ; the contraction being due chiefly to the cooling of 

 eruptive rocks and the dessication of sedimentary rocks. These 

 are, therefore, now properly known as contraction joints or shrink- 

 age cracks. 



The first class is by far the most universal and important. These 

 are the joints properly so-called of Dana and other writers. Many 



i Proc. B. S. N. H., Vol. xxn, pp. 72-S5. 



