120 G. M. Dawson — Earlier Cretaceous Bocks 



With such an amount of evidence there can therefore be lit- 

 tle doubt that pyroxene illustrates a hemihedrism in the mono- 

 clinic system, producing an apparent hemimorphism in the 

 direction of the vertical axis. 



Hessenberg's well-known figures of sphene crystals from 

 Rothenkopf in the Zillerthal, Tyrol, exhibit a similar hemi- 

 hedral development which he at first interpreted as hemi- 

 morphism in the direction of the clinodiagonal.* Yon Zeph- 

 arovich, however, subsequently showed that some of these 

 forms could be explained as twins,f and Iiessenberg also ex- 

 pressed himself as convinced that even the simplest of these 

 crystals were in reality contact-twins of the ordinary kind 

 where one of the individuals was reduced to an almost invis- 

 ibly thin lamella.:]: 



One of Hessenberg's figures is given on p. 385 of Dana's 

 System of Mineralogy (f. 372). I have been permitted 

 through the kindness of Mr. C. S. Bement, of Philadelphia, to 

 examine his fine suite of sphene crystals from the Rothenkopf, 

 which includes the finest crystal ever found there, figured in 

 the Neues Jahrbuch fur Min., etc., 1874, p. 828. These all 

 appear to be contact-twins, and it is probable that Hessen- 

 berg's second explanation of their apparent hemimorphism or 

 hemihedrism is the correct one. Nevertheless their peculiar 

 habit is of interest in connection with the hemihedral develop- 

 ment of pyroxene. 



Petrographical Laboratory, Johns Hopkins University, 

 Baltimore, March, 1889. 



Akt. XIY. — On the Earlier Cretaceous Rocks of the North- 

 western portion of the Dominion of Canada y by George 

 M. Dawson, Assistant Director Geological Survey of 

 Canada. 



In the Report of Progress of the Geological Survey of 

 Canada for 1872-73, the late Mr. James Richardson first de- 

 scribed an important series of rocks occurring in the Queen 

 Charlotte Islands, which Mr. E. Billings, on paleontological 

 grounds, in an Appendix to the same report, characterized as 

 probably lowest Cretaceous or Upper Jurassic. A further and 



* Mineralogische Notizen, No. 6, p. 19, PL II. figs. 26-34. 1864. Neues Jahr- 

 buch fur Min., etc., 1874, p. 828. If the position of Dana be taken instead of 

 that adopted by Rose and Naumann, as Hessenberg himself allowed to be more 

 rational, these crystals become apparently hemimorphic in the direction of the 

 vertical axis, like pyroxene. 



f Sitzungsber. Ak. Wiss. Wien., vol. lx, p. 815, 1869. 



% Mineralogische Notizen, No. 11, p. 19, PI. II, figs. 16-18, 1873. 



