A REMAKKABLE OCCURRENCE OF CALCITE IN SILK^I 



FIED WOOD. 



By Edgar T. Wherry, 

 Assistant Curator Division of Mineralogy and Petrology. 



D Wnl^n^ V ,7' ?'''''i'^ ^'"' ^""^^^^^ ^y ^^'^- Charles 



D A^alcott in lellowstone National Park during the summer of 



It IS dark brown in color and shows, even to the naked eye 

 unmistakable vegetable fiber. Dotted through the mass are minute 

 white grains, with rhombic crystal outlines, and with dark central 

 .V It fi 1 r '' ^'"'^ examined by Dr. F. H. Knowlton and 

 Identified as Sequoia magnifica Knowlton.^ The major part of the 

 replacing material is silica, but the crystal grains are calcite, and 

 l^dy^S^^^ ^-' - - ^" that a special 



When thin sections are examined under the microscope the posi- 

 ons of the ca cite grains are found to bear no definite relation to 

 the ^ood structure; tl>ey cut across the cells in all directions. Thev 

 are bounded by somewhat convex crystal planes; by measuring tht 

 angle between the trace of the vertical axis (lo;ated by the fnte " 

 ference figure in convergent polarized light), and these planes, in a 

 number of grains lying on their sides, the dominant form was deter 

 mined to be e, the half negative rhombohedron : observed ai-oc' 

 calculated 20= 15'. The base, ., is sometimes obscurely deveL'ed' 

 as ,s also the unit rhombohedron r. A few of the grains are S 

 orysta s, but the majority of them are formed by thf intergro,vth°„ 

 several crystals. The individuals appear always to be nterl iwn 

 in twinning position, the twinning plane being r, the unit rh^mbo 

 hedron, the vertical axes accordingly making .^rngies of 90" Ic'Tvith 

 one another, and e.xtinction in the two parts of the twin bein "It 

 times nearly simultaneous. This twinnino- i= „«..„ , I?" . 

 several thin lamellae making up .-' c^nrat pit" f urgi^I" wMl^ 



PROCEEDINGS U. S. NaTIONaI MUSEUM. VOL. 63-2M0. ' 



227 



