Geology and Mineralogy. 309 



certainly has been no period of metamorphism in the region of the 

 northwestern States since the beginning of the Primordial. 



University of Wisconsin, March 8th, 1877. 



2. Mteroteopical Petrography ; by Ferdinand Zirkel. Being 

 Volume VI of the series of Keports of the U. S. Geological Explo- 

 ration of the 40th Parallel, Clarence Kara, Gedogist-m-charoe. 

 Submitted to the Chief of Engineers, and published by order of 

 the Secretary of War under authority of Congrc 



►*e plates. Washington, 1876.— The gre 

 nt Range of the Rooky Mountains, inciud 

 iinity of the 40th Parallel, abounds 



> trachytes and i\ iated kimls. some of them closelv like 

 rocks of central and southeastern Europe; and Mr. King has done 

 well in looking to Europe for the study and description of his 

 specimens, and has been most fortunate in securing the labors of 

 Prof. Zirkel, the leading author in microscopic lithology. The 

 results are therefore excellent, and at the same time they give to 

 the American sn European use of the names of 



rocks. 



kinds of crystalline rocks and their microscopic distinctions. In 

 this chapter Prof. Zirkel states that in his descriptions he uses the 

 term "ground-mass" for the mass of a rock where it is distinctly 

 crystalline granular under the microscope, and " base" when there 

 is an amorphous paste not ei inder the highest 



magnifying powei _. except in many obsidians, 



crystalline minerals. 



, He remarks also on the evidence that the crystalline minerals 

 m the "base" were formed while the latter still had a flowing 

 movement, as shown by the minerals ranging in straight or wavy 

 Jines, and by their fractures and abrupt bends or displacements ; 

 hence the positions and forms of the crystals have been partly 

 determined by the flowing ; and hence,' also, the rock has not 

 undergone any metamorphic changes since solidification took 

 Place. Those rocks whose micro-fluidal structure is particularly 

 •tistmet are gene rich in broken crystals shiv- 



ered mto deta,- rragmeata 



Ahe feldspar-bearing igneous rocks * * 



»e«»0/«7*ca: granite, granite-porphyry, felsite-porphyry, rhy- 

 OJ yte, obsidian, pearlyte, pan ' 'ontaining no 



2>«>rtz, and often trith more or less pla>/io<'lase feldspar: syenyte, 

 augite-syenyte, quartzless orthoclase-porphyry, trachyte, augite- 

 trachyte.— (c.) < , e or less nephel- 



ite or hvche : Foyayte, miascyte, orthoclase-porphyry, phonolyte 

 containing nephelite). b i» rocks. 



I \ V^ e FELDSPAR OF THE PLAGIOCLASE OR TRICLIXIC SEfilES.— 



!«•) Containing hornblende: quartz-dioryte, dioryte, porphyryte, 

 hornblende -por | ! rtz-propylyte, hornblende- 



andesyt^andda, : •a-dioryte.-(c) 



