Geology and Natural History. 73 



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in the habit and arrangement of the crystals of the different 

 feldspars produced, while differing in the size of the crystals of 

 the various feldspars according to their composition. These 

 results- have an important bearing on the problem of texture and 

 granularity in igneous rocks. 



First, as to the habit of the feldspar crystals produced from 

 solution of the feldspar constituents without admixture of other 

 material. So far as can be determined by microscopical study of 

 the sections, the crystals are in most cases blade-like in form ; 

 that is, they are elongated plates. They vary, however, from 

 one extreme to another, being in some cases equidimensional 

 plates of extreme thinness, in other cases prisms, elongated in 

 one direction with the other two dimensions equal. The develop- 

 ment of these forms takes place in feldspars of various composi- 

 tions, and appears to be chiefly a function of the rate of crystalli- 

 zation and not of the chemical composition of the feldspar, 

 except as this modifies the viscosity of the solution. It is not 

 possible to recognize any fixed relation between the habit of the 

 crystals and the composition of the feldspar. This is, of course, 

 in accord with the well-known isomorphism of the feldspar group. 



The common mode of crystallization in these preparations is 

 that of spherulitic aggregations, more or less completely devel- 

 oped in spherical forms, 



The elements of the spherulites are bundle- or sheaf -like aggre- 

 gations of long, thin blades, which blades lie nearly parallel to 

 one another in the middle or narrower part of the bundle, and 

 diverge at the ends into fan-like or plumose forms. Several of 

 these bundles or blades cross one another at the middle, and 

 when there are a sufficient number of bundles, or when they 

 diverge sufficiently, a completely spherulitic aggregation results. 



In some cases a spherulite consists of bundles or prisms that 

 extend uninterruptedly from the center to the outer margin, the 

 rays of the spherulite being nearly straight. In other cases the 

 spherulite is a composite of divergent bundles shorter than the 

 radius, which have been added to one another as though new 

 plumes had started from the ends of earlier ones. 



In most cases the middle portion of the feldspar bundles con- 

 sists of stouter crystals than the outer parts. It also appears 

 that the middle portion is more prismatic, in certain cases some- 

 what cuboidal, the outer parts becoming delicately tabular. This, 

 with the divergence in position, explains the spread of the outer 

 part of the sphere. There is a great increase in the number of 

 individual crystals in the outer portion of the spherulite, and in 

 some cases the crystals also increase in size in the outer part. 



The shapes of the crystals are due to the flattening of the crys- 

 tal parallel to the second pinacoid (010), and its elongation paral- 

 lel to the crystal axis a. The outlines of the plates appear to 

 conform to traces of several pinacoids in the zone of the b axis, 

 (001), (201), (101), (201), (304), (203), not all of these occurring 

 together. It is quite probable that pinacoids in the zone of the 

 c axis also may be developed, but they were not recognized. 



