18 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.64 



with the Challis argentine in appearance and like it has the calcite 

 plates coated with thin reticulate layers of quartz. The Washington 

 argentine is associated with laumontite. 



The large argentine masses clearly are later, in age, than both the 

 mordenite and the spherulitic quartz and the calcite is older than 

 the reticulated quartz and the analcite, which occurs as flattened 

 crystals between the calcite plates. In the specimen shown in plate 

 1, lower, however, the calcite plates are embedded in the mordenite 

 and thin sections show that the calcite plates of this specimen not 

 only certainly are earlier than the mordenite, but that they are older 

 than the early heulandite deposit which lines the cavity and coats 

 the free plates of calcite on both sides. Elsewhere in the large 

 mordenite specimens there are flat gashes, now lined with analcite, 

 which are apparently casts left by the removal by solution of early 

 calcite plates. There are obviously two generations of the calcite 

 in the specimens, both of which have the unusual argentine habit. 

 The habit of the calcite may be due to the fact that the solutions 

 from which it crystallized were highly saturated with silica and the 

 calcite plates and their surfacing films of quartz are more or less 

 contemporaneous, the silica presumably having deposited in colloidal 

 form. 



CONCLUSION. 



The assemblage of minerals here described occurs as fillings of 

 cavities, often of very irregular form and large size, in what is prob- 

 ably a flow of rather glassy andesite with a porphyritic development 

 of augite, poured out in a Miocene lake basin. The cavities with 

 their fillings may be so closely spaced as to make up a large propor- 

 tion of the bulk of the rock. In general, the augite phenocrysts of 

 the andesite are quite unaltered and there is no evidence of extensive 

 alteration of the groundmass, so that it is evident that the minerals 

 of the cavities were not derived by the breaking down of any of the 

 constituents of the immediately adjacent rock. So far as can be 

 determined from a study of the specimens available the order of 

 deposition has been as follows, beginning with the earliest mineral 

 deposited: Chlorites (diabantite, etc.), chalcedony, calcite (argen- 

 tine), heulandite, mordenite, quartz (spherulitic), calcite (argentine), 

 quartz (films), analcite. 



The deposition of both quartz and chalcedony was apparently 

 repeated or was continuous over a considerable period. Of the 

 minerals, mordenite, a comparatively rare zeolite, is by far the most 

 abundant. The area deserves further study and may be expected 

 to produce other minerals of interest if carefully explored. 



