F. SLAVIK PRAHA~ UNIVEI~SI'I'Y CIIARLES 1Y

Note 0n the Pillow lavas

.kt tile Madrid International Geological Congress, in 1926. I communicated t) tile results of investig.~tions of the Algonzbm pillow lavas in tire Centratand Western BoHemia, made I)y ,~te from petrological point of view and comple- ted by Mr. V. V E S E L Y , j by chemical analyses. Tlte most char.'teteristie feature of the .X_lgonkian pillow lavas of Bo- hemia is the almost exclusively magmatie and vitreous n a -

t u r e of the interstitial mass between the , pillows ,; but only one [)lace, the rural street of Vodoehody Not' Prah~t, has yielded diabasic interstiti.tt glass which was sufficiently fresh ['or chemical aualyses. Whilst the re'tin mass o1' the , pillows , has been proved to be of normal basaH;ie (dia- basic) composition, the interstitial glass has been formed to be of uttrabasie, approximately pier[tie nature. So the pillow b~va of godochody is a rarely extreme case of che- mical di~Ferenciation in an eff~aive rock mass, and tile sense of that differentiation front more acid to more basic p..trtial magmas repeats in marry times greater size the phenomena in variolites z) attd some amygdaloid bas:tlts a). In all other (more than 20) BoHemian pillow lavas secondary alterations prevent to pursue this interesting question by further che- mical analyses; but microscopical observations, principally on the metamorplaosed pillov-lavas from the Zelegnd hory, also of i lgonkian age, show the same tendanee.

In the discussion following my eomtaunieation at the

1) S~,tvtu F. - - L e s ,( p i l l o w - la*,~a~r ~ a l g o T t k i e l t ~ z e s d e l a B o h g m e ,

Comptes rendus du X1Ve Congr~s Gdologique InteruationaL 2) LOEWtNSON- LESSING F. J . - Tscherm. Min. perf. Mittl. 1884

( v i ) , 281-3oo. 3) NEWTON E. T. and TEA/oh J. J. H . - Q. J. Geolog. Soc. 1897

(LIII), 485-488.

�9 74 �9

Madrid Congress, Mr. Logw~NsoN LESSlNG pointed out, that in the variolitic roehs of the Mugodjary Mrs. he also found a magmatie differentiation, whilst e. q. in the pillow ]avas of Crimea (Karadagh) the main mass and the inter- stitial substance are of the same composition.

The ~c pillow lavas )~ are developped in very many eru- ptive formations of basaltic composition, as Mr. d. Vo,.rc~:Y LEw~s 1) showed in his excellent mmmgraph. Therefore, I should like to draw the attention of' volcanologists and petrographs studying recent or extinct areas of basaltic magmas to this phenomena and to the importance ~f ga- therlng further materials .for i.~ve,tigation., on magmatic differeneiation in effusive rock ma,ses.

l) VOLNE~" LEwis J . - Origin of Pillow Lava,, Bulletin of the Geological Society of America. 1914, (XXXV), 591-654.