450 J. E. WOLFF; — GEOLOGY OF THE CRAZY MOUNTAINS. 



the contacts of the laccolites are striking. The rock has a coarse, often 

 granitic, texture in the middle, but becomes dense and porphyritic within 

 a few feet of the contact. The thinner sheets and dikes have throughout 

 their mass the character of the contact varieties of the corresponding 

 larger masses. 



Lithologic Characters. — Brief mention should be made here of the varie- 

 ties of eruptive rock thus occurring. The most prominent is a dark basic 

 rock found in all three forms (the laccolites reaching over 350 feet in a 

 single sheet), and having a coarse granitic texture in all but the dikes and 

 thinner sheets. This rock, originally found here by the writer in 1883* 

 was found to he composed of feldspar (in part triclinic), augite and 

 nepheline, with biotite, sodalite, magnetite, olivine, segirine, etc, acces- 

 sory; as an abyssal intrusive rock with the mineral combination nephe- 

 line, soda-lime feldspar, it filled a gap in the classification of Professor 

 Rosenbusch, and was called by him "theralite," as the first undoubted 

 representative of this family. 



Associated with the theralite in parallel sheets or dikes are lighter 

 colored alkaline rocks with a much higher content of silica, which in the 

 thinner sheets correspond exactly in mineral composition and structure 

 to the effusive rock called acmite-trachyte (often phonolitic) and in the 

 heavy sheets resemble some eleolite-syenites (c. g., those from Arkansas). 

 Other sheets and dikes composed essentially of triclinic feldspar, augite, 

 hornblende, or biotite appear to belong to various groups ( diorite-porphy- 

 rite, camptonite, etc). The completed petrographical study of all these 

 varieties is expected to bring out interesting relations between composi- 

 tion, structure and geological occurrence. 



Featukes of the Southern Area. 



The geology of the central mass of peaks in the southern half remains 

 to he described. The radial drainage and " fall-line " features of the 

 topography are due to the presence of a central mass or stock of coarsely 

 crystalline diorite and granite, which has hardened and metamorphosed 

 the Cretaceous strata for the distance of nearly a mile outward. The 

 streams which head within the area of crystalline rock have to cut 

 through this contact /.one or ring of hard rock, beyond which they have 

 cut deeper into the normal soft strata and widened their valleys. The 

 diorite stock is irregularly oval in outline and is about 6 miles wide at 

 the greatest diameter. The rock is composed of triclinic feldspar and 

 hornblende, biotite. augite, hypersthene, often quartz and orthoclase, 

 with the usual accessory minerals, but the composition varies somewhat. 



* Neues Jahrb., op. < - i t . 



