PRISMATIC STRUCTURE IN IGNEOUS ROCKS 233 



The second general type is produced by convectional circulation 

 of the magma while still liquid. The cells so produced persist until 

 solidification begins, and may leave a record in the rock either by 

 causing segregation in the cell walls and axes, or by originating 

 regularly spaced centers of crystallization. The experimental and 

 observational data on the occurrence of this type in igneous 

 rocks are suggestive, but cannot yet be said to amount to decisive 

 proof. 



A third type of prismatic structure is produced by internal 

 expansion. It has been produced artificially, and is offered as the 

 explanation of the "weather-crack" structure seen in diabase 

 bowlders. 



In the study of these structures, the following field observations 

 are those which will be of greatest interest in the further study of 

 the problem: (1) attitude of prisms, (2) their diameter and length, 

 (3) frequency of four-, five-, six-, and seven-sided polygons, (4) fre- 

 quency of angles (especially 90 and 120 ), (5) variation, if any, of 

 composition and texture in the cross-section, (6) types of cross- 

 jointing (platy, concave or convex, spheroidal), (7) spacing of cross- 

 joints, (8) peculiarities of cross- joints (e.g., whether cracked from 

 center or from borders), (9) degree of irregularity in sides of prisms, 

 (10) other peculiarities, such as tapering, partial' longitudinal joint- 

 ing, etc. 



The primary object of this discussion is to call attention to the 

 possibilities of the prismatic structure of a given rock body as an 

 index of its conditions of formation. Quantitative data on col- 

 umnar structures are very scarce; yet quantitative measurements 

 must precede quantitative deductions. We wish to know the tem- 

 perature of the rock when it was intruded or extruded ; its viscosity 

 when it began to cool and when it began to crystallize; the amount 

 and kind of gases which it released; if extrusive, the climatic con- 

 ditions under which it cooled; if intrusive, the properties of its 

 inclosing strata at the time of intrusion. These and other facts are 

 deducible only from the present properties of the rock, among which 

 its prismatic structure will prove of great importance. 



Equally necessary with the field data are experimental studies 

 of the structures produced in a cooling magma under conditions 



