lxvi GENERAL SUMMARY OF SCIENTIFIC AND 



scription of several species, including two new minerals, Ster- 

 lingite and Hallite, he discusses their optical properties, and 

 explains the well-known diversity in the angle of the optic 

 axes by a system of molecular twinning ; exceedingly inter- 

 esting and ingenious, if not very probable. He suggests the 

 application of the same principle to the micas, and to other 

 hexagonal minerals. 



Professor Dana has recently published in the American 

 Journal of Science an important paper on mineral pseudo- 

 morphism, descriptive of the changes which have taken place 

 at the Tilly-Foster iron-mine at Brewster, N. Y. No less 

 than eleven minerals are enumerated which have undergone 

 a complete change to serpentine, together with a variety of 

 other pseudomorphs of equally interesting nature. 



An elaborate paper by A. Schrauf, of Vienna, and Edward 

 S. Dana, treats of the thermo-electrical properties of various 

 minerals, and sustains the conclusion that these properties 

 are not dependent on hemihedrism, or peculiarities in crys- 

 tallization, but are connected, in some cases at least, with dif- 

 ferences in chemical composition, and always with differences 

 in density. 



The subject of micro-lithology continues to excite much 

 interest ; indeed, it is a question whether any kindred sub- 

 ject has at present more devotees. The elaborate works of 

 Boricky on the "Basalts and Phonolytes of Bohemia," and 

 Doelter on the "Trachytes of Hungary," of Allport and Hull 

 on the English igneous rocks, will show what is being done 

 to develop the subject of lithology. The large collections of 

 igneous rocks made by the Clarence King party have been 

 in the hands of Professor Zirkel, of Leipsic, and interesting 

 results may be looked for in the report when published. The 

 trap-rocks of the Atlantic border are now under investiga- 

 tion microscopically by E. S. Dana, and chemically by G. W. 

 Hawes ; and some of the results thus far obtained were 

 brought before the meeting of the American Association at 

 Hartford. 



The following is an enumeration of the most important 

 new species of minerals (see also above) which have been 

 described during the year: 



Dawsonite. An interesting mineral, from a trachytic dike, 

 near Montreal, and described by Mr. B. J. Harrington, of the 



