256 IRVING. 



present in great abundance. The former exhibit the usual au- 

 gite core, at times completely lacking in pleochroism, sur- 

 rounded by a border of a greener hue, until we have encircling 

 all, a rim of deep green pleochroic segirine. This outer border 

 often contains inclusions, and is the last formed portion of the 

 crystal. In the specimen from the sheet on the Burlington and 

 Missouri River R. R. above the mine the smaller needles of 

 segirine are much more infrequent, and the larger phenocrysts 

 are well developed, some of them attaining the size of y^ inch 

 in diameter. The only other mineral noticed in this rock was 

 biotite, which is seen sometimes filling the cavities left by the de- 

 composition of the larger crystals of aegirine to whose outline 

 it frequently conforms. It occurs chiefly in the rottener speci- 

 mens, and is probably an alteration product. 



6. Bald Mountain Type. 



A very peculiar variety of quartz-segirite-porphyry is that 

 exposed on the summit of Bald mountain. It is the most un- 

 usual rock that the writer has observed from the hills. 



Megascopic Appearance. — In the hand specimens it has a 

 rather dense texture, and a light greenish white color. Sanidine 

 phenocrysts are sparsely scattered through it, but rarely attain 

 the diameter of y^ inch. Quartz phenocrysts appear at rare 

 intervals. The main body of the rock is a light greenish white 

 groundmass with rough trachytic appearance, and closely re- 

 sembles a tuff" or sandstone. 



Aficroscopic Characters. — The microscope shows the following 

 minerals : Orihoclasc, quarts, plagioclase and cEgirine. 



The orthoclase phenocrysts show no unusual features, except 

 that they contain many inclusions of plagioclase and a few frag- 

 ments of earlier formed sanidine. The groundmass, however, 

 is extremely peculiar, as it is composed almost wholly of auto- 

 morphic quartz crystals of uniform size. These are square, 

 hexagonal, or triangular in sections and average .012 mm. in 

 diameter. They give all tests characteristic of the mineral. 

 The centers of the crystals are usually free from inclusions, but 

 as the border is approached a maze of extremely minute aegirine 



