550 THE DISTRIBUTION, ETC., OP ALKALINE ROCKS, 



Jura strata). All the highest summits are composed of trachytic 

 and andesitic tuffs, breccias and flows (cf. N. S.W. occurrences). 



The area of extrusive flows is defined by the 5000 feet contour 

 {cf. the Nandewars). The earliest flows were composed of ada- 

 mellose or trachy-andesite, the later of basalt (as in N. S.W. 

 areas). Many interesting types of alkaline rock have been 

 described from this region, as for instance shonkinite (Highwood 

 Minette), absarokite, and several varieties of monzonite. 



The differentiation-phenomena recorded in the paper are of 

 considerable interest, and are also discussed in Brogger's work on 

 the Eruptive Rocks of the Christiariia District, Vol. iii. 



The intruded rocks are considered to be of early Tertiary age. 



Nothing has so far been published on the petrography of the 

 Ortiz Mountains, New Mexico, but from a paper by Ida H. 

 Ogilvie* we learn that they form a high altitude conoplain con- 

 sisting of Cretaceous strata tilted by an igneous laccolite and 

 subsequently denuded. The intrusion is of Tertiary age, 



J. E. Wolff f has described certaiu nepheline rocks from the 

 Crazy Mountains, Montana. They form dykes in Cretaceous 

 .sandstone, and consist of augite, biotite, nepheline, plagioclase, 

 magnetite, olivine, and hauyne, being therefore allied to monchi- 

 quite and theralite. 



Wolff and Tarr have described acmite trachytes from the Crazy 

 Mountains, Montana. These rocks occur as dykes cutting the 

 Laramie as sheets parallel to the bedding, and as thick laccolites. 

 They are associated with theralites. 



From Texas nepheline syenites, phonolites and allied rocks 

 have been described by Osann.J The rocks occur in the Apache 

 Mountains. The Mount Ord Range contains true nepheline 

 syenite with a border phonolitic facies. In the Sawtooth Moun- 

 tains, west of the Apache Mountains, there is an alkaline syenite 

 which passes into rhomben-porphyry at the margin. The dyke 



* American Geologist, July, 1905. 

 t Neues Jahrbuch, Vol.i. 1885, p.69. 

 % A. Osann, Tscher. Min. Mitth. Vol.xv. p. 394. 



