W. Cross — Igneous Rocks in Wyoming. 141 



the planes of albitic twinning the feldspar is in part replaced 

 by cloudy isotropic matter leaving remnants of feldspar with 

 somewhat weakened double refraction here and there. In 

 some parts where no optical action can be discerned, the 

 former twinning planes may be traced by the varying cloudi- 

 ness. On the contact with the orendite containing this inclu- 

 sion, the cloudy mass gives way to a clear glass. In it are 

 apparently colorless rounded or irregular granules which seem 

 to be diopside, though not developed in determinable form. 

 The contact between inclusions and rock is sharp and no 

 change in the character of the latter can be made out which 

 might be referred to assimilation of the fused inclusion. 



Augite is much less attacked than feldspar ; it is often 

 entirely unaltered in contact with the cloudy product from the 

 feldspar. In some places the diopside grains have a rim of 

 apparent resorption origin, comparable with those so common 

 about hornblende. This zone has a granular appearance and is 

 usually not resolvable into distinct mineral constituents, but 

 where the grain affected is in contact with the surrounding 

 rock reddish-brown mica, magnetite, and a predominant pale 

 green pyroxene seem to be the resulting minerals. 



In the rock of Orenda Butte numerous pebble-like inclusions 

 were found which belonged principally to two types, one con- 

 sisting almost entirely of labradorite with a few specks of ferro- 

 magnesian minerals, and the other a granular mixture of quartz 

 and alkali feldspar. In the former a cloudy alteration of the 

 plagioclase penetrating between the grains and on cleavage 

 planes is like that already described. 



The quartz-alkali feldspar rock shows much greater altera- 

 tion. Megascopically these inclusions are seen to be very 

 irregularly porous, the feldspar dull and the quartz grains 

 much cracked. Under the microscope the feldspar presents 

 various intermediate stages of alteration from the normal min- 

 eral to a glass. The progress of alteration is marked by the 

 appearance of a cloud of minute dark bodies along basal and 

 pinacoidal cleavage planes and on chance fractures. These 

 seem under high powers to be gas pores and to be connected 

 by irregular arms where most numerous. The optical action of 

 the feldspathic substance is often distinguishable but much 

 weakened. A peculiarity observed in one of the inclusions 

 seems worthy of notice. It is the development of the apparent 

 gas pores on curving concentric planes comparable only to 

 those of typical perlite. This phenomenon is most distinct 

 and takes place in feldspar substance still showing optical action 

 though weaker than normal. Isotropic arms representing 

 complete fusion penetrate many grains following the cleavage 

 and other fissures marked by the cloudy parts. The recrystalli- 

 zation of the melted parts has yielded a tine, irregular spherulitic 

 growth in some places. 



