John LeConte—Joints in Clay dnd Marl Deposits. 288 
Art. XXIII.—Origin of Jointed Structure in undisturbed Clay 
and Marl deposits; by JoHN LeContE. (From a letter 
addressed to Professor J. D. Dana, dated Berkeley, Califor- 
nia, Jan. 10, 1882.) 
homena of immense vertical cracks or joints, evidently due to 
the shrinkage of these marshy deposits during the progress of 
their desiccation. These cracks, by éheir intersection, form 
tributary of the Sacramento). hen this stratum of clay is 
Subjected to the desiccating influence of the prolonged dry 
Season, cracks are fo hrinkage extending to the bed 
Powerful desiccating influence of the arid climate which suc- 
ceeded the Glacial epoch. 
It is evident that when such cracks or joints become filled, 
by subaerial agencies, with sand and other materials, the subse- 
quent lines of erosion must necessarily follow these channels 
