Tarr— Origin of the Chert in the Burlington Limestone. 441 



stem is not too long, the axial canal has been preserved by 

 becoming filled with silica. 



The fossils in the chert were included in it as it accumulated 

 on the sea bottom. Organisms fell upon or into the soft col- 

 loidal silica and were buried. The hard parts of the organisms 

 remained and the organic matter upon its removal gave rise to 

 cavities in the silica. The remains of Mastoids are almost 

 always hollow, but both valves of a brachiopod must be present 

 in order to form such a cavity on its interior. Some of the 

 corals are hollow but most of them are partly filled with chert. 



These internal cavities are lined with drusy quartz crystals, 

 a feature which is readily explained, because the cavity remained 

 open and was tilled with water and some silica. The silica 

 crystallized slowly from the solution and replaced the calcite of 

 the spiralia of the brachiopod and the septa of the corals and 

 then lined them with drusy crystals. Small crystals of sphaler- 

 ite, pyrite and chalcopyrite are found on the interior of a few 

 of these fossils. Salts of the metals when in solution readily 

 penetrate colloids, as has been shown by Tolman and Clark,* 

 and precipitation could have taken place on the interior by 

 means of hydrogen sulphide, which was formed by the decay 

 of the organic matter of the organism. 



Several specimens have been found in which the chert 

 entirely surrounds one of the Syringopora corals (see fig. 12). 

 The tubes pass entirely through the nodules. Evidently the 

 coral was surrounded by the colloidal silica and preserved. 

 This njode of occurrence is interpreted to mean that the coral 

 and the chert nodule were growing at the same time. It does 

 not appear probable that the coral would be so perfectly pre- 

 served in an upright position if it had simply been buried like 

 the numerous remains of the crinoids which apparently fell 

 into the silica. ]^ot a single occurrence has been found in 

 which this coral was preserved elsewhere than in the chert. 



Still another important feature is exhibited by the chert. 

 This is the imprint of corals, crinoids, and brachiopods on the 

 exterior of the nodules (see fig. 13). These imprints are very 

 perfect and show how the fossil was forced down into the soft 

 silica g'el. 



6. Absence of Siliceous Organisms in the Burlington. 



If siliceous organisms furnished the siHca for the chert, as is 

 claimed by those who think chert was formed by concentration 

 of silica from the surrounding rocks, evidence of this former 

 existence should be found in either the chert or the limestone. 

 A very careful search was made in the thin sections of the 

 chert for traces of such organisms, but none were found. The 



* Tolman, C. F., Jr., and Clark, J. D., Econ. Geol., vol. ix, p. 559, 1914, 



Am. Jour. Sci. — Fourth Series, Vol. XLIV. No. 264. — December, 1917. 

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