346 THE CANADIAN NATURALIST. [Vol. viil. 



bourhood of Achern and OfFenburg, but sometimes only in too 

 small a quantity to determine quantitatively. In the crystals of 

 the same rock from Carlsbad there is, according to Redner, only 

 about half a per cent. (0.48 along with 2.41 Na 2 and 

 15.67 K 2 0)*. The orthoclase, rich in potash, of this rock is 

 known to weather but very slowly ; as the crystals, loose and com- 

 paratively but slightly weathered, can in many places he picked 

 out of the grit into which the rock has crumbled. The baryta 

 separates out only when the crystals are much decomposed, and 

 this seems to take place to any considerable extent only locally. To 

 this corresponds the variety and narrowness of the veins of baryta 

 in the region of porphyritic granite from Achern to the Kinzig- 

 thal, where such are observed only on the high Horn near Zell, 

 by Schloss Staufenberg (7 cm), on the Lautenbaechle in the 

 Durbachthal (2J cm). Others near Gemsbach in the Murgthal 

 exhibit slighter dimensions. The only ores observed in them 

 were spathic iron and a poor limonite. 



Otherwise is. the behaviour of the non-porphyritic granite that 

 extends from Rippoldsau along the eastern side of the chief gneiss 

 zone of the Black Forest past Schapbach, Wittichen, Alpirsbach, 

 Schiltach, Hornberg and Tryberg towards St. Blasien. In the 

 northern part of this region the orthoclase is poorer in potash 

 (7.81 per cent.) but richer in soda (3.24 per cent.) and 

 contains along with 0.58 lime 0.22 per cent, baryta. f It 

 weathers a good deal more readily than that of the por- 

 phyritic granite, probably on account of its greater per 

 centage of lime and soda. Wherever the feldspar is mostly con- 

 verted into pinite and the rock is much loosened it is traversed 

 by innumerable barytes and metallic veins, which terminate 

 abruptly wherever the fresh granite replaces this porous rock. 

 The veins consist merely of a series of stringers, which quickly 

 unite and attain a width of half an inch to ten inches at the 

 most, only to separate again. Only towards the boundaries of 

 the gneiss, for instance, in the Tiefenbachthal, near Schapbach, 

 does this behaviour change, and the veins become far wider up 

 to five feet ; but with merely traces of cobalt fahlerz. Only 



* The entire composition of the feldspar is : Si 2 63.02, Al 2 3 



18.28, Mg O 0.14, Ba O 0.48, Na 2 2.41, K 2 15.67. 

 f Its entire composition, according to Nessler is : Si0 2 65.59, 

 Alj 3 20.53, Mg O 0.44, Ca O 0.58, Ba O 0.22, Na^ O 3.24, K 2 7.81. 



