THE ORIGIN OF CERTAIN SILICEOUS ROCKS 367 



an originally siliceous sediment of Griswold. The latter must 

 either have been chemical, which Griswold expressly rejects, 

 and apparently very properly, or composed of very fine granular 

 quartz. In the latter case the granules, however fine, have been 

 still further granulated, a species of metamorphism which has 

 apparently not been observed elsewhere, and which is difficult 

 to conceive. The metamorphism, if there had been any, would 

 rather be expected to be in the direction of larger and well- 

 defined grains of quartz, such as were noted in some of Griswold's 

 slides (No. 14) . x The latter slide seems to be a case of replace- 

 ment according to Rutley's hypothesis " caught in the act." It 

 may be suspected that in a larger suite, especially of non- 

 commercial stones, other such cases might be found. 



Griswold's explanation of the cavities seems also to point in 

 the direction of a replacement, and can be reconciled with 

 Rutley's hypothesis on the supposition, which does not seem 

 difficult, that part of the original calcareous sediment may have 

 crystallized (not necessarily as dolomite) and thus have offered 

 greater resistance to the replacement. 



The description of the beds also is suggestive of original 

 limestones with cherty layers and nodules. Two of the speci- 

 mens sent appear to me to represent such cherts, though I should 

 not call them metamorphosed. 



The question of the state of the silica seems to me to be one 

 of quite secondary importance, as in the case of silicified wood 

 replacement silica is met with in all states, and often with 

 an appearance of a passage to the state of quartz after the 

 replacement. 



Viewing the question from a distance, and on a very insuf- 

 ficient basis, it seems probable that if Mr. Griswold had taken 

 into consideration the hypothesis of a replacement of calcareous 

 sediments, he would, perhaps, have found much more in its 

 favor than either Rutley or I could see. 



For the study of crucial points, "non-commercial" rock will 



1 Whetstones and the Novaculites of Arkansas. By L. S. Griswold. Ann. Rep. 

 Geol. Surv. Ark. for 1890, III, p. 128. 



