778 PROCEEDINGS OF THE NATIONAL MUSEUM. 



ous, pitchy-black rock glass with conchoidal fracture and without 

 crystal developmeut of any kind. No. 338 is dark steel gray pitch- 

 stone with greasy luster, even fracture, and microperlitic-soiideriug. 

 In this rock the first evidences of crystallization are seen in parallel, 

 yellowish-brown bauds of microfelsite and in other minute crystallitic 

 ibrms, including some small spherulites. No. 133 is a light-gray banded 

 pitchstone porphyry, consisting of a groundmass of glass and microlel- 

 site in about equal proportion, and a few small phenocrysts of quartz, 

 feldspar, and mica. 



No. 343 is a light gray, beautifully banded rhyolite, differing from 

 No. 133 in the holocrystalline, spherulitic condition of the groundmass. 



The spherulites of this rock are about one-tenth of a millimeter in 

 diameter, very symmetrical in form, and arranged in approximately 

 parallel lines. They are separated by micropoikilitic areas of quartz 

 and feldspar, containing minute crystals of magnetite and biotite, and 

 by bands of microfelsite, which give the rock its characteristic banded 

 structure. 



The spherulites are very dense and are made up of radiating fibers 

 of an optically negative feldspar elongated parallel a; thus differing 

 from those found in the comj)ound spherulites from Aqua Dulce (No. 

 53285, original ^'o. 483) (see p. 779), which are uniformly positive in 

 longest dimension. This fibrous feldspar is not always confined to the 

 spherulitic bodies, but extends frequently into the adjoining poikilitic 

 areas, and there assumes the form of well-developed radiating crystals 

 with extinction angles of 10° to 12°. 



This relatively large angle is characteristic of soda orthoclase. 



Quartz could not be directly determined in the spherulites, but on 

 examination between crossed nicols, however, an anomalous divergence 

 in the arms of the dark cross would suggest its presence. Streams of 

 dark crystallitic grains, presumably magnetite, foUow the general par- 

 allel structure 6f the rock only deviating from their course to avoid 

 phenocrysts and some spherulitic forms containing feldspar nuclei. 



Peculiar spherulitic nodules are found in the obsidian flows (No. 

 53285) which, alternating with sheets of rhyolite, form a high bluff' on 

 the right bank of Aqua Dulce Creek close to the United States border. 



These nodular masses are thick-lenticular in shape, and with their 

 irregular botryoidal surfaces bear some resemblance to full-blown roses. 

 Some forms aie more spherical with less uneven surfaces and are not 

 unlike tulip bulbs. 



The spherulites are of a brown terra cotta color and vary from 1 to 2i 

 inches in greater diameter. They occur singly or in groups and always 

 with ribbed or net veined surfaces. These veins consist of secondaiy 

 quartz which has filled shrinkage cracks in the nodules and now stands 

 out in considerable relief. In thin sections it is evident that these 

 nodules, although apparently homogeneous in mineral composition are 

 structurally extremely complex. They are made up of an intricate 

 intergrowth of brown, finely fibrous spherulites composing the botry- 



