74 Scientific Intelligence. 



Bladed forms in some cases prove to be aggregates of thin 

 plates not strictly parallel to one another in the plane of flatten- 

 ing, so that the blade is curved and not straight in the direction 

 of its longest axis. 



In some spherulites the component crystals are prisms through- 

 out, with no tabular flattening. The number of crystal prisms 

 increases from the center of the spherulite outward by the devel- 

 opment of new prisms at slightly divergent angles, in arborescent 

 arrangement. 



The most complex arrangements are produced by twinning and 

 divergence combined, resulting in feather-like aggregates. Long, 

 narrow, tapering blades in albite twins form a shaft, elongated 

 parallel to the crystal axis a, on two sides of which diverge at a 

 slight angle a double set of thin blades, like barbs. These con- 

 sist of branched smaller blades or prisms, like barbules, the 

 branch prisms having approximately the direction of the crystal 

 axis c. The two sets in each " barb " are apparently related to 

 one another as the halves of a manebach twin. The small prisms 

 are composed of many subparallel plates flattened in the plane of 

 the second pinacoid (010). These correspond to barbicels in a 

 feather. 



With respect to the size of the crystals it is extremely signifi- 

 cant that pure anorthite (An) develops in comparatively large 

 plates, 5 mm thick and 20 to 30 mm long, in a few hours, whereas 

 the more sodic the feldspar the smaller the individual crystals 

 formed under almost the same conditions of cooling. Thus with 

 oligoclase (Ab 4 An x ) the individual crystals composing a bundle 

 of blades are considerably less than 0'01 mm thick, probably about 

 0-001 mm , a difference in thickness when compared with anorthite 

 of about 5,000 to 1. This as shown elsewhere is due to the 

 greater viscosity of the liquid feldspars near their solidifying 

 point as they approach the albite end of the series. 



Any comparison of the grain of rocks, that is, the size of the 

 constituent crystals, with a view to determining the physical con- 

 ditions attending the solidification of the magma, must be based 

 in the first instance on a knowledge of the behavior of the vari- 

 ous rock-making minerals under similar physical conditions, both 

 separately and in combination, that is, in solution with one 

 another. The granularity of rocks is clearly a function of the 

 chemical composition. 



With respect to the homogeneity of the crystals separating 

 from the liquid, it is observed that the great part of each crystal 

 aggregation appears to be of one composition, but that in some 

 cases a small proportion, probably less than 3 percent, is different 

 from the bulk of the feldspar, both in composition and habit. In 

 one instance this small variant differed in composition but not in 

 habit from the main mass of crystals. 



In the first case it appears that crystallization began with feld- 

 spar richer in the anorthite molecule than the solution and devel- 

 oped cuboidal forms. These were prolonged into prismatic 



