710 GEOLOGY OF OLD HAMPSHIEE COUNTY, MASS. 



the pit again, the large blocks of clay when dislodged would slip apart 

 easily along the planes of bedding, where the films of sand lessened the 

 cohesion, and expose broad surfaces of the tessellated pavement. 



It was very plain that the greater ease with which the moisture could 

 escape along these sand layers was the determining cause of the appearance 

 of tlie structure along these jilaues. The moisture escaped so gradually and 

 the clay was so nearly homogenous that the shrinkage tension could distri- 

 bute itself equally through(iut the mass and finally relieve itself by a system 

 of fissures at angles of 60° and 120°, of great mathematical regularity. 



President Hitchcock mentions^ three localities, one on the Agawam and 

 two near the Deei-field River, where these joints also occurred in the same 

 clays, and considers them to be due to a crystallization of the clay and "to 

 be a more simple operation of the sanae general cause which produced the 

 concretions." 



In an elaborate paper entitled "On the structure of rocks called joint- 

 ing,"" Prof W. King says: 



Hitchcock states that " unconsolidated clay beds iu West Springfield and Deer- 

 field, in Massachusetts, are intersected by numerous and distinct joints, while those 

 above and below are unaffected. This clay has certainly never been subjected to any 

 great degree of heat, being of very recent origin.'" It is to be apprehended that 

 there is some oversight in this statement. 



This seems to be a wholly groundless assumption on the part of the 

 author, made in support of the theory advanced in the paper cited. I may 

 add that the fissures extend vertically downward through the fat lamina? as 

 if cut with a knife, and pass down through the sandy laminte with a curved 

 surface. 



The torsion theory of Daubree will hardly apply, as the joints are 

 found in limited areas having relation to recent erosions, or in blufiFs pro- 

 duced by digging. I have searched the clays for many years for fossils 

 and concretions, and these joints have been wanting in so great a number 

 of cases where all the conditions were favorable that they can not well be 

 referred to any such general cause. All the cases occurred in bluffs where 

 the wall below was strong and well supported and there would seem to 



' Geology of Massachusetts, 1841, p. 418. 



'Trans. Eoyal Irish Acad., Dublin, vol. 25, p. 606. 



' Elementary Geology, p. 22. 



