50 G. K. Gilbert — Or'nj'rn of Joint* J Struct u,'r. 



Art. VI.— On the Origin of Jointed Structure; by 



In the March number of this Journal, President LeConte 

 proposes to explain the jomted structure of the Quaternary 



clavs of the Great Salt Lake Desert by referring it to the same 

 category wit! eeria - i dcam cracks observed in recent 

 (.'n'iitornian alluvial deposits. The cracks he describes form by 



rangular or hexagonal prismatic blocks.''— and these words 



cracks. The joints dividing the clays of the Desert have, on 

 the contrary, a regular arrangement. In describing them in 

 the January number of the Journal, 1 spoke of a drainage 

 system to which they give rise, and said that the blocks 

 marked out by that system are "rudely rectangular,'' but the 

 adjective rude could not with propriety be applied to the blocks 

 cut out by the joints, for these Melograms. 



The joints are definitely divided into two systems, one nearly 

 at right angles to the other, and within each system they are 

 parallel. For this reason I am led to regard the proposed ex- 



When a moist clay stratum shrinks by drying, its fracture 

 is resisted, first, by its internal cohesion, and second, by its 

 adhesion to that <m which it rests. The average size of the 

 b'.oidcs into which it. divide- is determined by these two condi- 

 tions — the cohesion tending to make them large, the adhesion to 

 make them small. The forms of the blocks are determined pri- 

 marily by the fact that the contraction is equal in all direc- 

 tion-, and we may conceive that each block tends to be circu- 

 lar about its center of adhesion — becoming 

 with as many sides as there are contiguous blocks." The 

 arrangement of the centers of adhesion follows no law, but is 

 ondit'mned in part b\ slight inequalities of adhesion or cohe- 

 sion, and is irregular. There is always some inequality in the 

 size of the blocks and the number of their sides ranges ordi- 

 narily from 4 to ,. It. is a general characteristic of the cracks 

 thus formed that they meet but do not cross each other. If 

 four come together in such way as to include equal angles it is 

 purely a matter of accident. Odinarilv only three meet in a 

 point and neither of the three is, properlv -peaking, the con- 

 tinuation of one of the others. 



j features characteristic of sun cracks in clay are repeated 



•erfieial layer of any material shrinks so rapidly 



:rated bv a great variety of cooling 



ng processes in the art.-, and conspicuously by the 



ck. SU $he 



