W. Cross — Topaz and Garnet in Rhyolite. 433 



N.W.-S.E. Nearly vertical planes of contact with gneiss are 

 shown in one or two places, but the valley alluvium surrounds 

 the masses for the greater part. Between the first and second 

 ridges and east of them are more or less stratified pink or 

 whitish rhyolitic tufa beds, containing bowlders of Archaean, 

 rhyolite, and other eruptive rocks. This tufa is no doubt geo- 

 logically connected with that occurring quite extensively 

 among the low Archaean hills on the east bank of the Arkansas 

 between Nathrop and Sali'da. The rhyolite ridges themselves 

 seem from present appearances to be short dikes. 



The Rhyolite. — The rock of the ridges mentioned varies some- 

 what in appearance, but cavities containing topaz and garnet are 

 .common to all, though most numerous and best developed in 

 that mass referred to by Mr. Smith, which will therefore be first 

 described. 



The greater part of the riclge in question is made up of a 

 white or grayish, more or less banded rhyolite, which is as a 

 rule so compact that no constituent minerals are recognizable. 

 The banded structure is produced by the alternation of light 

 and darker gray layers, and is occasionally emphasized by thin 

 bands of crystalline quartz or by an equivalent of the latter in 

 the shape of exceedingly flattened cavities, lined by crystals of 

 the same mineral. In this portion of the rhyolite no glass is 

 visible, but in certain places, particularly at the south end of 

 the dike and on its eastern side, there is a development of gray 

 pearlite, more or less cellular and usually containing round par- 

 ticles of a black obsidian, somewhat larger than a pea. Whether 

 these vitreous portions are contemporaneous with the banded 

 rock, or represent somewhat later injections of corresponding 

 magma, could not be definitely determined, owing to the debris- 

 covered slopes and the limited time available for their examin- 

 ation. It seems most likely, however, that they are merely 

 local phases of consolidation of the same magma as the compact 

 banded portions. 



Throughout both these types of rock are numerous cavities 

 which in many cases represent very well the peculiar vesicles 

 first accurately described by Yon Richthofen and called by 

 him 



Lilhophysen. — These are more or less round cavities, partially 

 filled by thin curved walls, which, by a concentric arrangement 

 and an overlapping, produce rose-like forms. In the present 

 case these folia are often not very well developed and appear as 

 low curved projections on the outer walls. Again, a cavity 

 may be nearly filled by a series of concentric shells. The outer 

 walls and the leaves of the calyx-like lithophyses are usually 

 lined by glassy quartz crystals of minute size, with prism and 

 pyramid. The former being clearly striated, the latter showing 

 the hemihedral forms quite evenly balanced. 



