198 WANLESS— LITHOLOGY OF WHITE RIVER SEDIMENTS. 



Calculated in terms of norms as described by Iddings,^^ this glass 

 is a rhyolite with the following standard mineral composition : 



Orthoclase 28.36 Diopside 2.38 



Albite 28.82 MgSiOs 2.00 



Anorthite 2.78 Ilmenite 0.60 



Quartz 31-93 Hematite 3.13 



In some specimens of these ash beds small cavities and irregular 

 cracks are lined with needle-like crystals of a zeolite, apparently 

 mordenite, of the composition (Ca, Na, K)2AloSiio024. 6^H20, with 

 a mean index of refraction of 1.465 and a very low double refraction, 

 about .005. This was probably formed as a result of slight hydro- 

 metamorphism by alkaline solutions deriving their soluble content 

 from the volcanic ash on which they reacted. There is little chance 

 of thermal metamorphism, as the ash should have been thoroughly 

 cooled after transportation several hundred miles in the air. The 

 cavities in which these zeolites have crystallized are not solution cavi- 

 ties, but more probably shrinkage cracks. The index of refraction 

 of the glass and pumice was found to vary between 1.495 ^^^ i-S^S- 

 The composition of the zeolite is very similar to that of the ash, and 

 it is probably a recrystallization of material derived from the glass 

 and pumice. 



In making the analysis of the ash, Professor Phillips found that 

 after being dried at 110°, to remove the absorbed moisture, on stand- 

 ing in the air the ash took up about 3 per cent, of water very readily. 

 This strong hygroscopic character is probably due to the attraction of 

 the capillary tubes of the pumice fragments for water, and may 

 account in part for the resistance to weathering of the ash beds, as 

 described above. 



The Clay Beds. 



The greater part of the White River formation is not made up of 

 the more interesting types already described, but rather of very fine 

 silt horizontally bedded and color-banded pale pink or brown and pale 

 green. These constitute all of the Oreodon and Titanotherium beds 

 except the nodular layers, channel sandstones, and limestone lenses. 



Their minerals are generally so fine grained as to be difficult of 

 determination, but fine angular fragments of quartz, more or less 



11 " Igneous Rocks," Vol. I., pp. 435 on. 



