244 JOSEPH P. IDDINGS AND EDWARD W. MORLEY 



biotite in the phanerocrystalline rocks of the Pic de Maros, while 

 augite is a prominent constituent in both series. The striking 

 contrast between the two groups of rocks is the crystalHzation of 

 leucite in the lavas, and the absence of nephelite, without any 

 considerable amount of orthoclase, which when present is a micros- 

 copic constituent of the groundmass, also the absence of biotite 

 in most phases of the leucitic lavas, although in some varieties 

 biotite and orthoclase have been crystallized at the expense of 

 leucite. While in the intrusive rocks of the Pic de Maros there is 

 no leucite, but considerable nephelite in some instances, abundant 

 orthoclase and biotite, and an absence of noticeable amounts of 

 lime-soda feldspar; in some rocks it is probably present molecu- 

 cularly in considerable amounts. These mineralogical contrasts 

 must be due to differences in chemical equilibrium within chemi- 

 cally similar magmas, resulting from physical differences attend- 

 ing the crystallization of lavas in one case and of intruded magmas 

 in the other. Such contrasts have been pointed out before, but 

 never with so good an illustration. 



A pseudoleucite kpntallenite, or fergusite, occurs with the 

 shonkinitic rocks in Celebes, the former leucites having been 

 replaced by orthoclase and nephelite. Leucitic lavas occur in the 

 neighborhood of the Pic de Maros, and in numerous other localities 

 in southwestern Celebes,^ and a mica-leucite basalt with very 

 similar chemical composition to the highly mafic shonkinite, 

 marosite, occurs in East-Central Borneo on the Oeloe Kajan.^ 

 Strongly potassic magmas yielding leucitic lavas, and biotite- 

 orthoclase phanerites, with more or less nephelite, occur widely 

 scattered from Eastern Java, through Southwestern Celebes, and 

 have been found in East-Central Borneo, so that it is probable 

 that other rocks of this kind will be found in the eastern part of 

 Southern Borneo. The extent of the region in which these leucitic 

 rocks are found, nearly i,ooo miles in length, is much greater than 

 that of Central Italy, which is at present the best-known region of 

 leucitic lavas. 



' H. 'Quc^m.g, Sammlungen des geol. Reichs-Museiim Leiden (Leyden, 1902), pt. 7. 

 45, and 1904, pt. 8. 



F. Rinne, Zeitschr. d.d. geol. Gesellsch., LII (1900), i. 

 ^ H. A. Brouwer, Versl. Kon. Akad. Wetensch. Amsterdam, 1909. 



