448 A. A. JULIEN AMPHIBOLE SCHISTS OF MANHATTAN ISLAND 



On this h3^pothesis also the following published analyses* of foreign 

 schists of the same mineralogical constitution as those of analyses V to 

 VII have a bearing : 



VIII. Hornblende schist. Milben, Germany. 

 IX. Hornblende schist. Treppenstein, Germany. 

 X. Amphibolite. Landeinwiirts, Germany. 

 XI. Hornblendite. Raiihenthal, Germany. 



XII. Palagonite tuff (average of 7 analyses), derived from basaltic ash 

 land. 



Ice- 





VIII. 



IX. 



X. 



XL 



XII. 



SiOa . . 



48.9 



46.7 

 2.8 

 4.3 

 8.0 

 18.1 

 14.8 

 2.0 



46.0 



""4.i" 

 12.3 



"10.3"" 



21.7 



46.4 

 6.7 

 4.6 

 2.1 



26.2 



""lo'.e" 



47.9 



T\02 





Al.Oa 



FeA 



FeO 



26.3 



" " ' 9.4 



10.0 



1.2 



I.O 



3.4 



13.5 



18.8 



CaO 



9.9 



MgO 



7.8 



K2O 



.8 



Na^O 



2.4 

 .5 







1.3 



H2O 



4.4 



3.6 













100.2 



99.6 



98.8 



100.2 



100.0 



These schists, VIII to XI, Rosenbusch separates from the igneous schists 

 of his analyses (see XXIX to XXXI beyond), considering them of sedi- 

 mentary origin. A comparison of their analyses with that of our horn- 

 blende schist shows wide divergence. 



DERIVATION FROM BASIC VOLCANIC ASH 



The other class may have been derived from layers of basaltic ash of 

 extreme basic composition. To alteration of such material Jukes t at- 

 tributed the hornblende schists observed by him in Ireland. 



In Germany, at Berggieshiibel, the beds of diabase tuff of the Lower 

 Silurian have been altered, in contact with granitite, into actinolite — or 

 anthophyllite — schist, often replaced with normal hornblende schist and 

 augite-hornblende schist. J However, all recorded analyses of these basic 

 tuffs (sideromelane, palagonite, etcetera,) § which are as rich in alumina 

 and iron oxides as the hornblende schist of Manhattan, show a decided 

 excess in silica, 4 to 7 per cent. The anal3^sis of a tuff || which I have 



* H. Rosenbusch : Tsch. min. u. pet. Mitth., vol. .\ii, 1891, p. 57. 



t student's Man. Geol., 1862, p. 169. 



J Roth: Allg. u. Chem. Geol., vol. iii, p. 167. 



g Roth : idem, vol. ii, p. :J01. 



II Roth : idem, vol. iii, p. 507. 



