352 G. H. Williams — Hornblende and its Gliding Planes. 



are capable of much more satisfactory explanation on a solid 

 globe than on an encrusted fluid one 



The only considerable novelty in the foregoing paper is the 

 proof that a simple strain spheroid affords an approximation to 

 the deformation of an elastic globe sufficiently close to serve 

 as a basis for Sir William Thomson's demonstration of the 

 rigidity of the earth ; but an endeavor has also been made to 

 present the whole subject in a clear and elementary manner. 



U. S. Geol. Survey, Washington, Feb. 1890. 



Art. XLIY. — On the Hornblende of St. Lawrence County, 

 N. Y., and its Gliding Planes ; by George H. Williams. 



A perfect cross-parting due to intercalated twinning lam- 

 ell 2d has been incidentally referred to by the writer* and by 

 the late Professor vom Pathf as occurring on certain crystals 

 of brown hornblende from Pierrepont, St. Lawrence County, 

 N". Y. The phenomenon is altogether analogous to the basal 

 parting much more commonly observed on pyroxene ; and, on 

 account of the general interest attaching to the gliding or struct- 

 ure planes of crystals, it seems deserving of further study. 



Hornblende is one of the important silicates whose occur- 

 rence in the great belts of crystalline limestone in northern 

 New York has rendered St. Lawrence County a well-known 

 mineral locality. From its composition this hornblende is to 

 be referred to actinolite. It presents two well marked varie- 

 ties, which however differ in hardly any other respect than 





(I) 



(II) 



Si0 2 



56*54 



56- 



44 



Ti0 2 









11 



A1 2 3 



1-10 



1- 



77 



Fe 2 3 



•69 





84 



FeO 



2'30 





73 



MnO 



-. — --, 





•11 



CaO 



13-69 



11 



83 



MgO 



24-42 



22 



•98 



Na,0 



1-15 



2 



13 



K 2 









75 



Ignition 







2*46 



99-95 



100-15 



color. The one of these is green and comes principally from 

 the neighborhood of Russell ; while the other is brown and 



* This Journal, III, vol. xxix, p. 486, June, 1885. 



f Sitzungsber. d. niederrh. Ges. f. Natur- und Heilkunde, Juli 7, 1886. 





