150 CARNEGIE INSTITUTION OF WASHINGTON. 



Sci. (4), 16, 151, 1903), viz, that natural bornite is of constant composition 

 represented by the formula CU5FCS4. 



(16) Tvpcs of prismatic structure in ifjiu-ous rocks. R. B. Sosman. J. Geol., 24, 215-234 

 (1916). 



From the physical standpoint, several types of prismatic structure in 

 igneous rocks can be distinguished. The first and most common is due 

 purely to thermal contraction in the crystallized rock; examples are numerous 

 and familiar. A subordinate type of contraction structure is produced when 

 the contraction and separation occur while the magma is still partly crystalline 

 and partly liquid; this type is illustrated by an occurrence in a diabase sill in 

 eastern Pennsylvania. 



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 crystalliza- 

 tion. The experimental and observational data on the occurrence of this type 

 in igneous rocks are suggestive, but can not 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 boulders. 



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 4, 5, 6, and 

 7 sided polygons; (4) frequency of angles (especially 90° and 120°); (5) varia- 

 tion, 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 peculiari- 

 ties, such as tapering, partial longitudinal jointing, etc. 



(17) Note on the linear force of growina; crystals. George F. Becker and Arthur L. DaJ^ 

 J. Geol., 24, 313-333 (1916)^ 



In 1905 the authors showed by appropriate experimental evidence that 

 a single crystal immersed in its own saturated solution, and growing by reason 

 of the potential supcrsaturation of the solution resulting from evaporation, 

 will lift a weight placed upon it. This observation has been confirmed in the 

 present paper. 



In 1913Bruhns and Mecklenburg placed two crj^stalsin a similar saturated 

 solution, one loaded and the other free, and noted that the load upon the one 

 crystal was not raised, although the free crystal grew rapidly. From this 

 experiment they were led to deny the power of a crystal to lift a weight of 

 foreign substance, although admitting the power of the unloaded crystal to 

 lift its own substance. They appear to have overlooked in this conclusion the 

 fact that the solubility of the loaded crystal is for most substances greater than 

 that of an unloaded one, and also that this is a difference in degree only, for 

 the unloaded crystal also supports weight (its own). 



In consequence of this greater solubility, with an unloaded and a loaded 

 crystal in the same solution, the necessary condition of potential supersatura- 

 tion will be reached in the liquid adjacent to the unloaded crystal before it is 

 reached in the other, and the growth of the unloaded crystal thereafter may 

 keep the concentration below that necessary for the growth of the loaded 

 crystal. This appears to be the condition reached in Bruhns and Mecklen- 

 burg's experiment. If it happens, however, that the rate of growth of the 



