6 PEOCEEDIlSrGS OF THE NATIONAL MUSEUM vol.73 



in late Pliocene or early Pleistocene time. No intrusive rocks of 

 assured late Pliocene or Pleistocene age are known in the region, so 

 that no close connection between the hot spring and intrusive rocks 

 can be shown. Doubtless the hot spring once reached the surface 

 several hundred feet or even more above its present outlet and the 

 zeolites were deposited by it. An attractive hypothesis assumes 

 that the minerals with increasing silica content were deposited dur- 

 ing a period of rising temperature of the waters and that the final 

 calcite in the fissure represents the latest mineral of the cooling stage. 



DESCRIPTION OF THE MINERALS 

 By Earl V. Shannon and Forest A. Gonyer 



DIAEANTITB 



In many of the specimens the first lining of the amygdaloidal 

 cavities consists of a very thin layer of deep olive green to greenish- 

 black material, of waxy appearance and dull luster, which resembles 

 a clay mineral. This dark mineral preceded the zeolites in all cases 

 where it occurs and lines some vesicles which contain no other min- 

 eral. The layer is seldom more than a fraction of a millimeter in 

 thickness, and the material is not present in amount sufficient for 

 analysis. The thickest crust of it observed lined the small cavity 

 containing the zeolite described as levynite. Under the microscope 

 the dark mineral shows a micaceous, aggregated structure and con- 

 sists of small plates either interwoven or arranged in fan-shaped 

 groups and spherulites. It has fairly high birefringence and green- 

 ish-yellow color. Plates on edge show parallel extinction with faint 

 plechroism in greenish brown parallel to the fibers and brownish 

 green across the plates. The elongation is positive. No interference 

 figure could be obtained, but since plates lying on the base are dark 

 in all positions between crossed nicols it is believed that the axial 

 angle is near zero and the mineral optically negative. The refractive 

 index is variable, but the mean index of most of the grains is between 

 1.590 and 1.600. 



Though this mineral resembles some of the high-iron varieties of 

 the clay mineral beidellite, the optical properties are somewhat 

 nearer the chlorites diabantite and delessite, which are commonly 

 formed in amygdaloidal cavities of basic igneous rocks in associa- 

 tion with zeolites. 



A single small specimen of a white glassy zeolite not in amount 

 sufficient for analysis could not be satisfactorily identified, although 

 it may, wholly or in part, consist of levynite. The properties ob- 

 tained for this mineral are somewhat contradictory and its positive 

 identification must await the finding of additional material. 



