Explanation of Plate XXXIII. 



Fki. 1. Group of thinolito crystals from Marlile Biittes, Pyramid Lake (reiliuod ouo-lialf); open porous 



variety. 

 Kkjs. a an<l ;J. Transverse sections, natural size; Fig. 2, open skeleton I'orni ; Fii;. :!, jiartially tilled up 



with amorphous CaCOa. Theso sections show the system of rectangular (s(iuare) anil diagonal 



ribs, which consist of granular crystalline CaCOa. 

 Fia. 4. External appearance (reduced one half) of a single crystal, with part of a second, the Internal 



structure of which shows tliat it has but a single tennination ; the comparatively smooth surface 



is due to the secondary deposition of CaCOs. 

 Fig. 5. Longitudinal seclion of open vaiiety (reduced one-half), showing the two systems of plates 



converging upward at an angle of about 3.'j''. 

 Fig. 6. Complete crystal (reduced one-half) which yielded the section in Fig. 2; the line in which the 



section was made is indicated. 

 Fig. 7. Acute pyramidal crystal (reduced one-halt) which yielded at its base the section given in Fig. o. 

 Fig. H. Scpiare pyramidal crystal (reduced one-half) which gave, at the point indicated, the sectiou in 



Fig. 13 ; the surface has been made smooth by snbse(iueut deposition of CaCOa. 

 Figs. 9 anil 10. Skeleton crystals (natural size) showing cap-in-caii structure, and thus revealing the 



true square pyramidal form of the original mineral. 

 FlU. 11. Crystals (u-atural size) from the Domes, Pyramid Lake; tUe surface smoothed over by subse- 



qneut depositions of CaCO;i, with sproiitings from the edges and extremities. 

 Fig. I'i. Section (magnitied 8 times) of a crystal from the Domes, like tliat in Fig. 11, showing a 



diagonal and rectangular frame-work, partly erjstalline, granular, partly amorphous, with layers 



of secondary caibonate opal-like in structure. 

 Fig. 13. Section (natural size) of the crystal shown in Fig. 8, cut transver.sely at point indicated; it 



shows the same frame-work of granular crystalline carbonate, partially lilleil in with secondary 



CaCO:,. 

 Fig. 14. Section (natural size) .showing the usual frame-work, partially tilled in with secondary C'aCOa, 



and with successive layers also around the outside. 

 Fig. 15. Section of a crystal from the Marlde Buttes, magnitied H times, and showing the strnctnre lines 



of crystallized carbonate, and also in the cavities the acicular crystals of aragonite. (?) 

 FiG8. 1(5 and 17. Small pyramidal crystals (natural size), showing by dissection the cap-in-cap struc 



tnre, and thus, like Figs. II and 10, revealing the true pyramidal form of the original mineral. 



