392 Scientific Intelligence. 



the intervening spaces being open ; this is illustrated in fig. 10 r 

 which is a section cut across the crystal shown in fig. 1 at the point 

 indicated. In the more compact varieties the spaces between the 

 skeleton ribs, which uniformly consist of granular calcite, are 

 more or less completely filled with amorphous calcite, often in 

 concentric layers; this is shown in fig. 11 cut from the base of 

 the crystal shown in fig. 8, and still more in fig. 12 cut at the 

 point indicated from the crystal shown in fig. 9. A longitudinal 

 section of a crystal resembling fig. 1 is shown in fig. 4 and reveals 

 a system of plates converging toward one extremity at an ap- 

 proximate angle of 35°. 



The arrangement of the system of plates of which these pseu- 

 domorphous crystals are made up, as shown in the figures, proves 

 that the crystals conformed approximately to the type of an 

 acute square pyramid. The structure of numerous dissected crys- 

 tals, as shown in figs. 2, 3 and 5, which are not uncommon in 

 some of the specimens, conforms to this type ; as does also the 

 external form whenever distinct. Provisionally then it is con- 

 cluded that the original mineral, after which the thinolite is a 

 pseudomorph, crystallized in the tetragonal system in acute pyram- 

 idal forms. The crystals might equally well be referred to an 

 orthorhombic or monoclinic form approximating to the tetragonal, 

 but the simpler hypothesis seems the more probable. 



Only a negative conclusion as to the nature of the original 

 mineral is definitely reached, viz : that it could not have been 

 gay-lussite, nor gypsum, nor anhydrite, nor in fact any known 

 mineral. It is suggested, however, in view of the fact that the 

 crystals resemble in form the pseudomorphs of lead carbonate 

 after phosgenite (PbC0 3 + PbCl 2 ), that the original mineral in 

 this case may have been an isomorphous salt with the composi- 

 tion CaC0 3 + CaCl 2 , or perhaps CaC0 3 + 2NaCl. The value of 

 such a hypothesis will have to be tested by synthetic methods. 



In conclusion the writer calls attention to the resemblance of 

 many of the thinolite forms (cf. fig. 6) to the well known barley-corn 

 pseudomorphs of Sangerhausen, long referred to gay-lussite, al- 

 though that explanation was shown by DesCloizeaux to be errone- 

 ous. It was this resemblance which chiefly led Mr. King to refer the 

 thinolite also to gay-lussite. Pseudomorphs of calcium carbonate 

 apparently identical with those from Sangerhausen have been 

 described from half a dozen other localities, and it is suggested 

 here that probably all of them, and the thinolite also, were derived 

 from the same original mineral. The ultimate solution is there- 

 fore much to be desired on this account, as well as because it is so 

 important in explaining the geological history of Lake Lahontan. 



3. Address be/ore the Geological Section at the recent meeting 

 of the British Association at Aberdeen, by the President of the 

 Section, Prof. J. W. Judd. — In his able address at Aberdeen, 

 Prof. Judd reviews historically, first, the problem of the Scottish 

 Highlands ; secondly, that of the Elgin sandstone ; thirdly, the 

 relations between Scottish and Scandinavian geology ; fourthly, 



