

J. S. Diller — Origin of Paleotrochis. 339 



weathered most of the nodules become white as if kaolinized, 

 while the other nodules and the matrix remain quartzose in 

 appearance. The nodules form at least two-thirds of the mass 

 of the rock and are arranged with their longer diameters paral- 

 lel, rendering the rock rather easily split in one direction. 



With a lens, it may be seen that the small kaolinized nodules 

 exposed in section upon the surface of a hand specimen have a 

 radial fibrous structure. The same structure may be seen in 

 some of the larger ones, and in addition to this feature some 

 of the nodules possess a more or less distinct concentric shell- 

 like structure. These structures are usually best displayed 

 upon or close to a weathered surface. Portions of the nodules 

 or spaces between them are in a few cases cellular, and the 

 walls of the openings are rarely lined with minute crystals. 

 The supposed fossil forms usually appear conical or discoidal 

 upon a weathered surface. They often show a small cup in 

 the apex and are surrounded by a narrow depression from 

 which the radial fibrous envelope pointed out by Mr. White 

 has been removed by weathering. 



A careful comparative study of the nodules in the hand 

 specimens tends to convince one that however different in form 

 and size the supposed fossils and concretions may appear, all 

 belong to one series and have essentially the same origin. 



A microscopical study of thin sections of the rock reveals 

 the fact that the nodules are spherulites, a common feature of 

 acid igneous rocks. They are composed in most cases chiefly 

 of fibrous feldspar with quartz or tridymite. As seen in the 

 thin section of the Paleotrochis-bearing rock, the fibers are 

 grouped radially with more or less irregularity in tufts, sheaves, 

 sectors, hemispheres or spheres. When they form a complete 

 sphere, which is rarely the case, they are most coarsely fibrous 

 or granophytic at the center and usually show between cross 

 nicols an indefinite black cross. Occasionally also the concen- 

 tric structure is well marked. The rays are too minute to 

 permit of an accurate determination of their mineral composi- 

 tion by optical methods, but microchemical tests with hydro- 

 fluosilicic acid yield the small cubic crystals, characteristic of 

 potassium fluosilicate as well as the hexagonal prisms of sodium 

 fluosilicate. Judging from the greater abundance of prisms 

 than cubes the fibrous feldspar is richer in sodium than potas- 

 sium. That feldspar, instead of chalcedony, is the most prom- 

 inent constituent of the spherulites, is fully borne out also by 

 its kaolinizing under the influence of the weather. 



The spherulites are embedded in a matrix composed chiefly 

 of granular quartz. The grains are occasionally so large that 

 the uniaxial positive character can be readily determined. 

 Untwinned feldspar in small grains may be present in con- 



Am. Jour Sol— Fourth Series, Vol. YII, No. 41.— May, 1899. 



