H. 8. Washington — New Leucite Rock. 35 



very loosely attached, so that the rock is very friable and 

 the leucite crystals are easily detached and the specimen 

 is readily crumbled to fragments between the fingers. 



Some small (up to 1 mm. long), sharp prisms of black 

 pyroxene are seen, that lie for the most part in the 

 cement, but also a few in the leucite crystals ; and there 

 are fewer, smaller black grains of what proves to be a 

 garnet. No other constituents are visible in the small 

 specimen in my possession. 



Although anticipating the description of the micro- 

 scopical characters, it may be said here that, while the 

 specimen suggests a tuff in its mineral composition and 

 friability, yet the warped surface just mentioned, and 

 especially the presence of glass as a cement and the 

 occurrence of pyroxene prisms partly in the glass and 

 partly in the leucites, with the texture as seen under the 

 microscope, leave no doubt that the specimen is from a 

 flow, and not a fragment of one of the somewhat consoli- 

 dated leucitic tuffs, which have been reported from this 

 volcano. Indeed, it may be suggested that such masses 

 of "tuff," or at least some of them, are in reality flows 

 of this highly leucitic lava, which has been mistaken for 

 tuff because of its highly friable character. 



Microscopical characters. — Unfortunately, only one 

 thin section, and this not a large one, was available for 

 study, two others having gone to pieces because of the 

 friability of the material. 



The largely predominant and anhedral, though gener- 

 ally equant, leucite crystals are perfectly clear and are 

 obviously extremely fresh. They show the character- 

 istic twinning in a most remarkable way, almost the 

 whole area of most of the crystals shoAving between 

 crossed nicols very numerous, differently oriented, sharp 

 lamellae, in some cases very thin, in others broad. In- 

 deed, among the numerous leucite lavas of which I have 

 studied thin sections, I can recall none that shows the 

 twinned structure so beautifully. 



The leucites are almost free from inclusions, and it 

 may be remarked specially that there is none of the zonal 

 or radial arrangement of small inclusions that is usually 

 so characteristic of this mineral. As inclusions occur 

 some small, anhedral pyroxenes and a few grains of 

 garnet (melanite), with rare, small, slender prisms of 

 apatite. A small area in one of the leucite crystals is 



