DISCONFORMITV UXCOXFORMITY 821 



starch cited, it belongs properly under the subject <jf diagenetic al- 

 terations. Thus there are at least three ways in which prismatic 

 structure is produced : 



1. Contraction and shrinking, on cooling or drying. 



2. Expansion and pressure by heating from without. 



3. Expansion and pressure by swellings from hydration. 



2.2. Insolation Joints. These are joints produced in massive 

 rocks, such as granite, etc., by the alternate expansion and contrac- 

 tion to which they were subjected under the diurnal heating and 

 cooling. Such joints are parallel to the surface subjected to change 

 in temperature, and are close together in the outer portion of the 

 mass, but farther apart at a depth. They serve an excellent pur- 

 pose in quarrying operations, which in such rocks would be more 

 difficult otherwise. 



D. Structures in Part Due to Deformation and in Part to 



Erosion. 



2^. Disconformity and Unconformity. Strata separated by an 

 unrepresented time interval are generally spoken of as unconforma- 

 bly related. Two types of such unconformable relation may be rec- 

 ognized, the stratic where the stratification of the formation on both 

 sides of the plane of nonconformity is parallel or nearly so, and 

 the structural, where the two sets of strata are inclined at a greater 

 or less angle with reference to each other. 



For the first type, in which no folding of the older set of strata 

 is involved, the term disconformity has been proposed (Grabau-6: 

 534), with the corresponding limitation of the term unconformity to 

 the second type, or that in which folding plus erosion of the first set 

 of strata precedes the formation of the second set. 



Crosby (2a) has called attention to the unsatisfactory character 

 of the prefix dis, since it means divergence rather than parallelism. 

 He prefers to divide unconformity into para-unconformity (parun- 

 conformity) — the disconformity of Grabau, and clino-unconformity 

 (clinunconformity) for the angular type with discordant strata. 

 Heini (9) had previously proposed the term paenaccordan:: for 

 approximate conformity with the strata nearly parallel. This, as 

 Crosby says, is not quite the equivalent of the parunconformity, 

 which implies crustal oscillation, rather than deformation, whereas 

 Heim's term suggests rather gradation between true conformity or 

 accordans, and unconformity or discordant. 



