21 B GEOLOGICAL HISTORY OF LAKE LAIIONTAN. 



follows: (1) The deposition of crystals as the lake waters evaporated; (2) 

 a change of conditions, e g., an addition of fresh water to the lake (as 

 supposed by King), leading to the solution of a part of the substance of tlie 

 crystals and the simultaneous recrystallization of the remaining calcium 

 carbonate; (3) the subsequent and independent deposition of the car- 

 bonate, solidifying and coating over the skeleton forms."" The conclusion 

 reached by Mr. King, that the original mineral was gaylussite, satisfies the 

 requirements tolerably well, for it is then necessary only to explain the 

 removal of the sodium carbonate, and the calcium carbonate remains 

 behind. Unfortunately for this hypothesis, it is impossible to reconcile 

 forms which now remain, showing how the original mineral crystallized 

 with the monoclinic forms of gaylussite."" Furthermore, Mr. Russell finds 

 several other gi'ounds, independent of this crystallographic proof, for the 

 belief that the supposed enormous deposit of gaylussite could not have 

 taken place. But if not gaylussite, what was the original mineral? 



"It is hardly profitable to go beyond the above suggestion, that it 

 may have been a double salt, containing CaCOg, unless the hypothesis can 

 be based upon some observed facts; but fortunately some facts can be 

 pointed to which lead to a possible explanation of the enigma, and which 

 are in any case very suggestive 



" The only crystalline forms bearing any close resemblance to the acute 

 tetragonal pyramids of the thinolite, of which the writer has any knowledge, 

 are those of the pseudomorphs of lead carbonate after phosgenite, first described 

 by Krug von Nidda,'' from the zinc mines in Upper Silesia. This simi- 

 larity in habit and angle is the more striking, as the thinolite form is an 



•»U. S, Geological Exploration of the Fortieth Parallel, Vol. I, p. 517. 



™At the time wheu Mr. Kiug had this subject under investigation he submitted several speci- 

 mens to the writer for inspection, and he then gave a qualified assent to the conclusion Mr. King had 

 reached in regard to them. One of these sjiecimens, as Mr. King had noted, showed some crystals 

 which bore a remarkably close resemblance to the well-known Sangerhaiiseu pseudomorphs, then gen- 

 erally referred to gaylus^site. This similarity suggested identity of origin — a conclusion which (after 

 a further study of the same specimen) the present investigation has confirmed, as noted below — iiud 

 thus gave apparent support to the gaylnssite hypothesis. The other specimens then in hand were 

 somewhat like Fig. 1, ou Plate XXXIII, and upon the inspection given them— noopport\inity was had for 

 careful study — -they gave negative results; a certain outward similarity to the elongated crystals . if 

 gayhLssite from South America (calh'd clavoa, nails) was noted, but nothing more definite. 



" Krug von Nidda: Ueber das Vorkonnnen dcs Hornbleierzes und des Weissbleierzes in den Krys- 

 tallformen des orsteren in Oberschlesieii, Zeitsch. geol. Gesellsch., II, Vil't, 18.')0. See also BInm, 

 PBeudomorphoseu, Zwcitiu' Naihtrag, 68. 



