286 



IRVING. 



spathic. In this are embedded phenocrysts, of plagioclase and 

 quartz, with occasional sanidines. The groundmass contains 

 scattered grains of magnetite. 



The plagioclases show an extinction angle on albite lamellae 

 of 1 8 to 20 degrees, which show them to be of quite a basic 

 type. They are sometimes seen in parallel growth with san- 



idine. In the only large 

 sanidine crystal seen in the 

 slide, a complete crystal of 

 plagioclase was included. 

 (Fig. 14.) 



The quartz is in extremely 

 large crystals, and abounds 

 in inclusions. It is resorbed 

 to a remarkable degree, the 

 groundmass encroaching on 

 it in unusually deep bottle- 

 shaped cinbayvicnts. 



It is very much fractured, 

 and frequently in long crys- 

 tals that have a far greater 

 lencrth than breadth. 



Plagioclase included in sanidine from 

 quartz-asgerite-porphyry from sheet on Bur- 

 lington and Missouri River Railroad, near 

 Terry Station. 



6. Diorite Family. 



Tonalite ( Quartz - mica 



hornblende - diorite ). — This 

 rock is exposed in the cut 

 of the Fremont, Elkhorn and Missouri Valley Railroad in 

 Deadwood Gulch, some little distance west of Go-to-Hell 

 Gulch, and is in an exceedingly thick dike in the Algonkian 

 formation. 



Megascopic Characters. — The rock is gray, granular-looking 

 and of an even texture, the constituent minerals being of such 

 size as to be easily recognized without the aid of the microscope. 

 Basic segregrations and angular inclusions of a basic character 

 are very numerous. 



Microscopic Cliaracters. — Under the microscope we may rec- 



