﻿RHYOLITE IN CEBU. 



409 



neglecting only the water lost at 110°, to about 2.4. Second, the quartz 

 and feldspar do not occur in well-formed crystals, but in sharp, angular 

 grains with the quartz often cracked, as if the siliceous waters forming 

 the precipitate had carried a small amount of clastic material. However, 

 fig. 1 and Plate I, fig. 1, show that reentrants are common and the absence 

 of crystal forms may be explained by magmatic corrosion. 



The small isotropic rods may be either casts of > or precipitates around 

 algae, 2 or they may be crystallites formed in the glassy base of an igneous 

 rock. They seem to bear a greater resemblance to the latter. The 

 banding ma} r be explained as either the result of original deposition or 

 as a flow structure. 



Below are tables of analyses of different rocks for comparison and a 

 recast of the analysis of this rock and classification by the quantitative 

 system. I have not been able to find any analysis of siliceous, precipitates 

 which at all corresponds to this rock, the nearest being a pulverent sinter, 

 No. 9. It will be seen that the rock agrees closely with Nos. 2, 3, and 

 4 and falls into the same subrang in the quantitative system. 



Comparative table of analysis. 



Constituent. 



1. 



2. 



3. 



4. 



5. 



6. 



7. 



8. 



9. 



Si0 2 



76. 51 



76.41 



72.68 



74.58 



75. 20 



73.91 



99.47 



81. 95 



94.63 



AloO ( 



14.93 

 .27 



14.42 



.48 

 .74 



15.62 

 .95 

 n.d. 



13.31 



1.31 

 n.d. 



12.96 



.37 

 .27 



15. 29 



.89 



1 17 



6.49 

 JTrace. 



1 



Fe,0 3 



FeO 



I 15.95 



MgO - 



1.24 



■ .24 



Trace. 



.26 



.12 





.50 



.15 



Trace. 



CaO 



1.-40 



1.43 



.63 



1.61 



.29 



. 77 



.90 



.50 



1.00 



Na-,0 



.67 



.63 



1.17 



1.52 



2.02 



3. 62 



.15 



2.56 



.30 



K»0 



4.98 



3.88 

 1.02 

 1.40 



4.30 

 2.90 



4.87 

 2.03 



8.38 



".58 



4.79 

 1.19 



.07 

 .12 



.65 



7.50 



1.02 

 7.43 



H»0^ 



CO.. 





Total 















100.00 



100.65 



100.02 



99.49 



100.19 



100.46 



101.50 



99.80 



99.97 



Specific gravity, 2.229. 



1. Cebu No. 57, neglecting H 2 0. 



2. Porphyry, Kiiserngrat, Windgalle Mountains, Switzerland. Schmidt C. : 

 Neu. Jahrb. B. B. (1886), 4, 432. (Washington, H. S.: U. 8. G. 8., P. P. 14, 

 130.) 



3. Rhyolite, Nagy-Mihaly, Hungary. Murokozy K. : Fbldtani Kbzlbny Buda- 

 pest (1892), 22, 54. (Washington, H. S.: U. 8. G. 8., P. P. 14, 130.) 



4. Porphyry, Arolo, Lago Maggiore, Piedmont. Ricciardi L. : Att. Acad. Gioen. 

 (1885), 18, 14. (Washington, H. S.: U. 8. G. 8., P. P. 14, 132.) 



Nos. 2, 3, 4: Persalane i Golumbare, Alsbachase, Mihalose, 

 ( Quarfelic, Domalkalic, Dopotassic. 



5. Rhyolite, Silver Cliff, Colo. Cross: Col. Sci. Soc. (1887), 229. (Kemp, 

 J. F.: Handb. of Rocks (190G), 28.) 



"-Weed, W. H.: V. S. G. 8., 9th An. Rep. (1888), 667. 



62926 3 



