254 Field Columbian Museum — Geology, Vol. i. 



mens which I took from the Pillar I find them to be made up chiefly 

 of aragonite. Not only is the specific gravity that of aragonite (2.92) 

 as obtained by Thoulet's solution, but several cavities show the typi- 

 cal radiating bladed crystals of this form of carbonate of lime. The 

 occurrence, therefore, furnishes an exception to the rule noted by 

 Merrill* that the onyx marbles are generally calcite. Between the 

 distinctly fibrous layers of some portions are interposed other layers 

 microgranular and non-fibrous in structure. The substance of these 

 I found to be of lower specific gravity than that of the fibrous layers. 

 It is in other words, calcite. Here, then, are variations from arago- 

 nite to calcite taking place in the growth of a single mass represent- 

 ing corresponding variations in the circumstances of its growth. A 

 similar occurrence is noted by Senftf in a deposit near Eisenach, 

 Germany. It is unfortunate that our present knowledge of the con- 

 ditions bringing about the formation of these two salts is so inade- 

 quate that we cannot know exactly what changes are indicated by 

 such alternations. 



Age of the Pillar. — The immensity of this stalagmite, and the 

 certainty that it has been formed by a fairly uniform process of 

 deposition, lead almost irresistibly to an inquiry as to whether any 

 satisfactory estimate of the length of time required for the forma- 

 tion of the mass can be made. Some attempts seem to have 

 been made to determine the rate of deposition by measuring the 

 thickness of the film formed upon glass vessels left in the water 

 now dripping at the Pillar. Unfortunately these measurements 

 are not very accurate. Collett states on ^ne page of his report 

 (p. 467) that water dripping "at the ' Pillar of the Constitution ' has 

 deposited a film of less than one-fiftieth of an inch during five years, 

 or at the rate of one inch in 250 years," while on another page (p. 

 474) he states that " an estimate based on quasi observations places 

 the rate of this stalagmitic growth at one inch in 100 to 150 years." 

 Hovey, in his "Celebrated American Caverns" (p. 138), speaks of 

 the Pillar as growing ten inches in 1,000 years, though he gives no 

 data on which to base the statement. Mr. Rothrock, the present 

 proprietor of the cave, has at my request had a new vessel placed 

 in the water since my visit and it is hoped that this may fur- 

 nish a means of accurate measurement in a few years. For the 

 present, however, taking Collett's lower rate of one inch in 250 years 



*The Onyx Marbles: their origin, composition, etc., Rep. U. S. Nat. Mus., 1893, p. 553. 

 tO/. cit., p. 289. 



