OEIGINAL CHAKAGTEE OF THINOLITE. 217 



unusual one. A number of these pseudomorphs are in the Blum collection, 

 which became tlie property of the Yale Mineralogical Museum in 1872. 

 They correspond to the description given by Krug von Nidda. They have 

 the form of a square prism, souietimes terminated by a pyramid having an 

 angle over the extremity of about 36°, and occasionally show traces of an 

 octagonal pyramid ; other forms show only a very acute square octahedron, 

 with a summit angle of about 13° in one case and 2(j° in another. One 

 specimen shows these forms imbedded in a white clay. They are now com- 

 pletely altered to compact, fine-granular lead cai'bonate, except for the pres- 

 ence of an occasional minute nucleus of the original mineral. 



" The hypothesis to which this resemblance leads is this : that the orig- 

 inal mineral may have been chloro-carbonate of calcium isomorphous with phosge- 

 nite ; that is, a mineral having the composition CaCOs-j-CaClg isomorphous 

 with PbC0a+PbCl2, and now altered to CaCOj, as in the phosgenite to 

 PbCO;j. The hypothesis, as far as the crystallographic relations are con- 

 cerned, is a most natural one. The difficulty arises when we consider the 

 peculiar nature of calcium chloride, and hence question whether an anhy- 

 drous molecular compound of calcium carbonate and calcium chloride 

 could have been deposited from the waters of Lake Lahontan. Obvi- 

 ously this is a subject for synthetic experiment, and whatever the nature of 

 the original mineral, it ought to be possible to approximate to the condi- 

 tions under which it was made and so to reproduce it. It is to be lio^ied 

 that the work now being carried forward by the chemists of the Geological 

 Survey may lead to some decisive results in this direction. 



" In the meantime it is interesting to note the only case in which, so 

 far as the writer can ascertain, a chloro-carbonate of calcium has been 

 formed. The experiments are described by Fritzsche in the Bulletin of the 

 St. Petersburg Academy for 1861, and reprinted in the Journal fiir prak- 

 tische Chemie.^" He states that on evaporating the solution of crystallized 

 calcium chloride, prepared in large quantities for technical purposes, there 

 remained a small amount of a sandy powder, which kept a yellowish aspect 

 so long as the calcium chloride solution was concentrated, but in a dilute 



"J. Fritzsche: Ueber ein Doppelsalz ans kohlensaurem Kalk uud chlorocalciuni. Jour, prakt- 

 Chem., LXXXIII, 21(), 1861. 



