BY H. I. JENSEN. 54ij 



It is interesting to note the correspondence in age and char- 

 acter of these intrusions with those determined by E. C. Andrews 

 in his New England Geology,* but of still greater interest is 

 Daley's theory that all these igneous irruptions are derived from 

 a more deep-seated magmatic reservoir, in which a huge basic intru- 

 sion has by stoping in and solution assimilated Palaeozoic sedi- 

 ments and subsequently differentiated. 



The Leucite Hills of Wyoming,! the chief rock-types of which 

 (wyomingite and orendite) have the composition of monchiquite, 

 stand in the midst of the Red Desert and form a huge dome, 

 approximately 30 by 40 miles, constituting perhaps an arid-cycle 

 conoplain like the Ortiz Mountains, U.S.A., and the Warrum- 

 bungle Mountains, N. S.W. From this area about 5000 feet of 

 Laramie beds have been removed by denudation. The elevation 

 of the dome was subsequent to the deposition of the Green River 

 Shales. The mass consists of necks, dykes, intruded sheets, 

 flows, etc.; some of the laccolites are now represented by mesas. 



The nature of the rock does not seem to be very varied, being 

 everywhere a somewhat basic leucite rock. 



A striking point is that the Red Beds were laid down on a 

 submerging land-surface of considerable relief J just as the Trias- 

 Jura beds of the Warrumbungle area and the Cretaceous beds 

 west of the Warrumbungles were laid down on a similar sub- 

 merging continental area. In both regions ancient granites and 

 crystalline rocks were exposed before the period of sedimentation. 



In the "Petrography and Geology of the High wood Mountains, 

 Montana,"§ L. V. Pirsson has shown that these mountains form 

 a group of eroded volcanoes rising out of almost undisturbed 

 Cretaceous strata (cf. the Warrumbungles in N. S.W., and Glass 

 House Mountains, Q., rising out of almost undisturbed Trias- 



* Records Geol. Survey of N. S. Wales, Vols. vii. and viii. 

 t Kemp and Knight, Bull. Geol. Soc. of Amer. Vol. xiv. pp. 305-336. 

 t Bull. Geol. Soc. of Amer. Vol. xvi. pp.205-214. 

 §Dept. of the Interior: U.S. Geol. Survey; Bull. No. 237, also Amer. 

 Journ. of Sci. Vol. xx. July, 1905. 



