1835.] Porphyritic Greenstone Rocks. 275 



frequently. It constitutes, in North Ural, the greater por- 

 tion of the main ridge, and forms the Konschekowskoy 

 Kamen, at Bogoslowsk, and the Belaja Gora, at Nischne 

 Tagilsk. Very distinct varieties occur at Alapajeusk, and 

 in the neighbourhood of Miask. 



In other countries, distinct mixtures form the diorite of 

 Rothenburg am KifFhauser, in Thiiringia, which is fre- 

 quently large grained ; of Ebersbach, and of the gigantic 

 pillars in Odenwald ; of Ehrenberge, at Ilmenau, which 

 contains, also, quartz and mica; of Hodritsch, near Schem- 

 nity, where pinchbeck-brown mica and flesh-red felspar oc- 

 cur, and where the considerable silver mine is worked; of 

 Guambacho, in Peru, of which an excellent specimen exists, 

 in the collection of Baron Humboldt. Distinct varieties 

 occur among the refuse of the mines, and among the rolled 

 masses, at Berlin, which contain milk-quartz as an accidental 

 constituent. 



Diorite, with hornblende predominating, occurs likewise 

 frequently in Ural, especially in the vicinity of Nischne, 

 and Werch-Isetsk, near Katharinenburg. It is also ob- 

 served in the Hartz at Rosstrappe ; at Mahnberg, on the 

 Ocker ; at Mitweida, in the Erzgebirge, and in many other 

 places which it is unnecessary to enumerate. 



2. Dioritic Porphyry consists of a mass containing albite 

 and crystals of hornblende enclosed. 



The main mass has, in the different varieties, partly a 

 greenish or blackish gray, partly a greenish or blackish 

 white, but dull colour ; anun even, fine, splintery fracture, 

 and is so hard, that it can with difficulty, or not at all, be 

 scratched with the knife. Before the blow-pipe it melts 

 into a black ish^green glass .'^ 



The albite occurs in white, shining, distinctly cleavable, 

 twin crystals, which exhibit distinctly the angles of perfect 

 cleavage faces. In some cases the crystals touch each other 

 sharply ; they are of a greenish or grayish colour, and have 

 a faint splintery fracture. In other instances they protrude 

 so slightly out of the basis, that they are only visible when 

 the mass is moistened. 



* Hose has made no trials to determine the nature of the basis of this and' the 

 following porphyry, but he supposes that it may consist of the substances which 

 are found crystallized in it. 



T 2 



