48 BULLETIN OF THE UNIVERSITY OP WISCONSIN 



I, Quartz keratophyre from Miihlenthal zw. Elbingerodeand Riibeland, 



Harz. (Jacobs ) ' 



II. Quartz kerat(){)hyre from Baraboo, Wis (Daniells.) - 

 in. Quartz keratophyre from Pigeon Pt., Mian. (Bay)ey )' 



IV. Quartz keratophyre from Baraboo, Wis. (Austin ) 



V. Quartz keratophj re from Mb. Elizabeth, Australia. (Howitt.)^ 



It is seen that the Baraboo rock compares very well in 

 composition with the quartz keratophyre of other regions. 

 The large amount of soda as compared with potash in 

 this rock, as shown by the analyses, indicates the presence 

 of a feldspar whose composition corresponds with a soda- 

 orthoclase. The excess of soda over lime in the Baraboo 

 rock also indicates that the plagioclase present contains a 

 large per cent, of the Ab. molecule. The specific gravity 

 of the orthoclase and plagioclase as determined by the 

 Thoulet solution was found to be about 2.63, both varieties 

 being apparently the same. This high specific gravity of 

 the feldspars is explained by the large amount of soda in 

 their composition, and in part, also, by the particles of 

 oxide of iron which they contain. 



A macroscopic description of the several types of rock 

 as they appear in the different parts of the eruptive area 

 has already been given. As the microscope shows the 

 structure of these types to be essentially the same both as 

 regards the minerals of the first generation and the ground- 

 mass, the phenocrysts and accessory minerals will first be 

 described, and then the various structures of the ground- 

 mass. 



Feldspar Phenocrysts. — Feldspar constitutes almost the 

 only porphyritic mineral. Soda-orthoclase and plagio- 

 clase are about equally abundant. Sometimes both va- 

 rieties are equally distributed in the same slide, and at 

 other times a single variety only is present. Microcline 



1 Quoted by Zirkel, Lehrb. d. Petrog , 2te Aufl , Vol. II; p. 334. 



2 Irving, Geo), of Wis., VoK II; p. 515. 

 SBayley, Am. Jour. Sci , (3) Vol. XXXVII; p Gl. 



■« Howitt, Roy. Soc. Victoria, 18S3, p. 25. Quoted by Rosenbusch, Min. u. petrog. Mitth. 

 VoL XI; p. irr. 



