NO. 2200. CALCITE IN 8ILICIFIED WOOD— WHERRY. 229 



less affected than those without, indicating that the growth of the 

 calcite took place fairly early in the history of the specimen. 



That the calcite was the first mineral to form is shown by the facts 

 that its grains possess crystal outline and inclose only woody matter, 

 never silica. When calcite crystallizes from solutions contained in 

 the pore spaces, between grains of sand it frequently incloses the 

 latter, yielding "sand calcites," df which many occurrences have 

 been described. In the present instance the Avood, in the process of 

 deca}'^, evidently became saturated with a solution capable of de- 

 positing calcite, and wood cells were inclosed, much as are the sand 

 grains in the " sand calcites." 



The only reasonable hypothesis which suggests itself to account 

 for the outer portions of the crystals excluding wood cells instead 

 of including them is that of decrease in rate of crystallization. It is 

 well known in the stud}'^ of crj'stallization that the rapid formation 

 of crystals renders them particularly likely to inclose mother liquor, 

 gas bubbles, or foreign particles suspended in the solution; slow 

 formation, on the other hand, furnishes opportunity for the grow- 

 ing crystals to push aside such obstacles. According to this view 

 the solutions percolating into the decomposing wood were at first 

 well saturated with calcium carbonate, and when crystallization was 

 inaugurated — perhaps by warming, by changes in pressure, or by re- 

 moval of carbon dioxide through the agency of bacteria — it went 

 ahead at first at a relatively rapid rate. The wood cells were at this 

 time included in the crystals. 



As the solutions became depleted in calcium carbonate, however, 

 the rate of crj'^stallization naturally slowed down and exclusion of 

 the cells took place. In time growth ceased, and a rim of cell ma- 

 terial surrounded the crystals. Then a slight renewal of calcium 

 carbonate in the solution occurred, and a thin layer of crystal matter 

 deposited over the surface of the previously formed crystals; this 

 also excluded most of the cell matter in its path, but traces remained 

 behind to mark the level of temporary cessation of growth. Silica 

 solutions then flowed in, and all remaining cavities were filled by 

 quartz. 



EXPLANATION OF PLATES. 



On the plates are shown photomicrographs of thin sections of the wood con- 

 taining calcite ci'ystals. 



Plate 29. 



(Enlai'ged 10 times.) 



Shows simple and twinned crystals, different forms of inclusions, the lack of 

 Influence of the wood structure on the directions of crystal gi-owth, and the lack 

 of distortion of the wood cells by the crystals. 



