592 
PETROLOGY: IDDINGS AND MORLEY 
ture. If this conjugate net has equal Laplace-Darboux invariants, a 
particular class of surfaces analogous to isothermic surfaces is defined. 
A projective generalization of geodesies may be made in terms of the 
congruence y^, since^ there exists a two-parameter family of curves on 
the surface whose osculating planes contain the lines y^. It must be 
possible also to generalize a good part of the theory of triply orthogonal 
systems and families of Lame, although the generalization can never be 
complete on account of the essential differences between metric and 
projective space. The field seems, on the whole, to be very promising. 
1 Green, G. M., Trans. Amer. Math. Soc, New York, 17, 1916, (483-516). 
2 Wilczynski, E. J., Ibid., 8, 1907, (233-260). 
3 Idem, Ibid., 9, 1908, (79-120). 
* Green, G. M., Amer. J. Math., Baltimore, 38, 1916, (313). 
' Darboux, Bull. Set. Math., Paris, (Ser. 2), 4, 1880, (348-384). 
6 Cf. the abstract of Miss P. Sperry, Bull. Amer. Math. Soc, New York, 22, 1915-1916, 
(441-442). The normal congruence is there replaced by the directrix congruence of the 
second kind, whose developables, however, do not cut the surface in a conjugate net. 
A CONTRIBUTION TO THE PETROGRAPHY OF SOUTHERN 
CELEBES 
By J. P. Iddings and E. W. Morley 
BRINKLOW. MARYLAND AND WEST HARTFORD. CONN. 
Communicated August 20, 1917 
In a paper in the Journal of Geology, Chicago, 23, 1915, (231-245), the 
authors described some rocks collected in Java and Celebes in 1910. 
The chemical analyses of seven of these were from lavas and coarsely 
crystalline igneous rocks occurring in the neighborhood of Bulu Saraung 
(Pic de Maros) . The rocks analyzed are trachytes, absarokite, nephelite- 
syenite and fergusite, besides kentallenite and marosite, rocks related 
to shonkinite. 
In November, 1914, a more extended visit was made to Southern 
Celebes under the auspices of the Bureau of Mines of the Netherlands 
Government. The mountainous region from Maros to Malawa and 
Batuku was studied in company with Mr. 'T Hoen and Mr. Ziegler, 
geologists of the Bureau. The region visited consists of several nearly 
parallel ranges of volcanic mountains, whose lavas are underlaid by 
faulted and dislocated strata which are exposed in the valleys and along 
the base of the volcanic ridges. 
The faulting and dislocation of the limestones and coal-bearing shales 
antedated the eruption of the igneous rocks, for the distorted strata 
are overlaid by volcanic breccias which form much less disturbed beds 
