162 The American Naturalist. [February, 



MINERALOGY AND PETROGRAPHY. 1 



Petrographical News.— Several contributions to the subject of 

 the origin of spherulites have recently been made by Messrs. Cross and 

 Iddings, and one on the minerals occurring in hollow spherulites by 

 the latter gentleman and Penfield. Iddings" distinguishes two kinds 

 of spherulites ; one composed of radial fibres forming the compact 

 spherulite ; and the second consisting of jointed and branching fibres 

 of feldspar, separated by tridymite scales and gas cavities. Grada- 

 tions between small, dense spherulites composed of micro-felsite, and 

 large ones, the nature of whose structure can be determined, were traced 

 in many instances, and from them the conclusion is reached that micro- 

 felsite is in many cases but a microscopic intergrowth of feldspars, 

 elongated parallel to the clino-axis, and quartz, and that the spherulites 

 are but special phases of granophyric growths. The discovery of tour- 

 maline and mica, especially near the margins of spherulites, is an addi- 

 tional proof of the correctness of Iddings' view that spherulites are 

 the result of crystallization of pasty rhyolitic magma under the influ- 

 ence of moisture. These two minerals are younger than the smaller 

 compact radial spherulites of the rock, and older than the final crys- 

 tallization of the residual magma between the spherulites. In the por- 

 ous spherulites with branching fibres, or the lityophysae, some of the 

 fibres are negative and others positive in the nature of their double 

 refraction ; that is, some are orthoclase crystals elongated parallel to c, 

 with the plane of the optical axes normal to the plane of symmetry, 

 and others are elongated parallel to a, with the plane of the optical 

 axes in the plane of symmetry. The essential characteristic of spheru- 

 litic growth is the internal structure of the spherulites. They are not 

 made up of amorphous substances under a strain, but of definitely 

 crystallized minerals arranged radially with one or several centres of 

 crystallization. Under this head, according to the author, would fall 

 granophyric intergrowths, which are radially branching aggregates of 

 orthoclase and quartz. Cross 3 places emphasis on the valuelessness of 

 the term microfelsite in petrographical. nomenclature, as he finds the 

 material to be an aggregate of quartz and orthoclase, two definite min- 

 erals, and not the ill-defined substance described by Rosenbusch. He 

 attacks both Rosenbusch's and Levy's classification of spherulites as 

 incapable of covering the handsome bodies found by himself in the 



1 Edited by Dr. W. S. Bayley, Colby University, Wateryille, Me. 



^Bull. Phil. Soc, Washington, xi, p. 1 B. 



