432 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



They also contain delicate needle-like prisms of augite, speckled with 

 magnetite grains. These radiate from the center of the spherulite and are 

 curved and forked, or branch into fern-shaped groups, with the branches 

 making an angle of about 36° with the main stem. Some spread in fan-like 

 forms; others branch at nearly 90°, the branching prisms being different 

 leno-ths. The cross sections are those of augite prisms bounded by prism 

 faces and a pinacoid. The outlines are often indistinct, as though the 

 prisms consisted of a number of parallel crystals. The extinction angle 

 reaches 42°. 



It is evident, both from the occurrence of large fragments of basalt 

 completely surrounded by rhyolite and from the microscopical character 

 of the rocks, that the rhyolite fused the basalt and was the more recent 

 eruption. The rhyolite must have been in a completely fused condition 

 when it came in contact with the basalt, so that its temperature was higher 

 than that of the melting point of basalt. The latter was not melted at any 

 considerable distance from the contact with rhyolite, but behaved as though 

 dissolved in the rhyolitic magma along the contact. 



