STRUCTURAL AND STRATIGRAPHICAL RELATIONS 455 



gone, no segirite rock.s and no minettes. In Middle Bntte we have similar 

 conditions. The principal laccolith, M. 1, is a diorite porphyry, and 

 among the sills and dikes are types all the way from trachytic to basaltic 

 affinities. We only know of one minette, sill 39, on the south side. No 

 fegirite rocks have been gathered. Although some of our specimens were 

 felsitic in texture and greenish in color, the slides showed the green stain 

 to be due to chlorite, not to gegirite. 



As we come to East Butte and take up the slides of its main laccolithic 

 summits, E. 1 and E. 2, segirite in its characteristic green needles makes 

 its appearance. The rocks have the familiar light green color of the 

 tinguaites and lead the observer in the field to suspect at once rich soda 

 and high alkali types. In the sills the trachytic and andesitic porphyries 

 ■ (respectively syenitic and dioritic) do not fail, but instances rich in 

 orthoclase and approximating trachytic composition are more frequent. 

 The extreme of the soda-rich segirite rocks among those in our collection 

 was obtained from a pipe or plug estimated by George M. Fowler, who 

 gathered the specimen (number 135), to be 10 feet in diameter. A more 

 complete description, with an analysis by H. S. Washington, will be given 

 below. Minettes also are more abundant in East Butte than in the other 

 two. The sills, numbers 46 and 48, in the Colorado shales, along the 

 valley south from the Strode ranch, in the northwest corner of the uplift, 

 are both minettes. Of 46 an analysis by Dr. Washington is later given. 

 Dike 133, 6 miles north of the main uplift, is the minette collected by 

 Dr. George Dawson and described petrographically by Weed and Pirsson, 

 as earlier stated. 



The rock of the laccolith having the two summits, E. 1 and E. 3, is 

 porphyritic in texture, with phenocrysts at times of relatively large size. 

 Under the microscope they are predominantly orthoclase, although plagio- 

 clase does not entirely fail. The orthoclase is often zonal and may reach 

 a centimeter in diameter. Earely quartz has been observed large enough 

 to be placed with the phenocrysts and favoring the borders of orthoclase. 

 The phenocrysts may attain such abundance that the groundmass appears 

 as if it were relatively narrow channels amid islands. The larger dark 

 silicate is augite, not always satisfactorily fresh. Its peculiar green sug- 

 gests the presence in it of the aegirite molecule. Away from the lower 

 contacts it is not abundant, but it does increase appreciably in the speci- 

 mens taken along the northern, lower border of the laccolith. 



The groundmass is microgranitic and consists of feldspars, quartz, and 

 the tiny green needles of aegirite. The feldspar is predominantly un- 

 tAvinned and is probably a soda-bearing orthoclase. It favors square cross- 

 sections to a degree worthy of remark. In instances extinction directions 

 XXXII— Bull. Geol. Soc. Am., Vol. 32, 1920 



