392 



F. EUTLET ON THE MICROSCOPIC CHARACTERS 



not very numerous) are, in some if not in all cases, orthoclase. 

 Some are perfectly developed ; others present the most ragged and 

 irregular boundaries, as in the accompanying figure (fig. 1). In 



i t Felspar Crystal in Obsidian (black), from the Yellowstone 



District. (Magnified 55 times, crossed Mcols.) 



this instance the spicular projections are sufficiently long and 

 delicate to completely invalidate the supposition that the crystal 

 is a broken fragment, and to render it absolutely certain that it 

 has been developed during the cooling and solidification of the rock 

 in which it occurs, or else that it is a partially dissolved crystal. 

 This is an important piece of evidence, because less irregularly 

 bounded imperfect crystals might often be erroneously regarded as 

 included fragments taken up by and enveloped within the lava. 

 Moreover it is interesting to see how, in such a remarkably homo- 

 geneous matrix, this crystal, if aborted, fails to assume the globular 

 form, rounded at the angles, which is so common in the porphyritic 

 crystals developed in most vitreous rocks. Under an amplification 

 of 300 or 400 diameters numerous clear granules and opaque tri- 

 chites are visible, as shown in fig. 1, PI. XX. ; and there are many 

 small objects which may be elongated gas-pores; but, as they mostly 

 pass diagonally through the section from the upper to the lower 

 surface, it is not easy in all cases to ascertain their true character, 

 owing to the impossibility of bringing them into focus throughout 

 their entire length. Those which coincide with the planes of section 

 certainly appear to be elongated, and sometimes tortuous, gas-pores. 

 The trichites are sometimes straight, sometimes curved, and frequently 

 form stellate groups. 



No. 2. Yellowstone district. Obsidian. 



Reddish-brown or Indian -red coloured rock, with some dark brown 

 or black streaks and mottling, opaque, except in thin sections. 

 Bright vitreous or slightly resinous lustre, and imperfect conchoidal 

 fracture. 



M. It is seen bytransmitted light (fig. 2,Pl.XX.)to consist of delicate 

 bands or strings of a clear yellow or orange-colour, with here and there 

 a few bluish-black or grey strings. The material lying between these 

 coloured strings, which are very closely packed, is a colourless or 



