1835i] and Porphyritic Greenstone Rocks, 445 



the last are much more imperfect and have a fatty lustre. 

 On account of the existence of the second faces of cleavage, 

 the diallage in the coarse granular varieties of gabbro 

 seldom splits in large pieces, but in plates, which are 

 not elastic, and are thus distinguishable from mica ; they 

 are frequently irregular and curved. Often the grains of 

 diallage exhibit a regular outline, and form symmetrical 

 hexagons with the same angles as in the most perfect faces 

 of cleavage of hypersthene. The colour is a dirty green 

 passing into gray, brown and black, often greenish and 

 grayish-white ; the complete faces of cleavage have a 

 metallic, pearly lustre, the others are faint, or have a fatty 

 lustre. The fusibility of diallage before the blow-pipe is 

 very small ; it melts on the platinum forceps in thin splinters 

 on the edges into a blackish-gray splendent glass.* The 

 fragments of diallage in Gabbro are more frequent and 

 more distinct than in the hypersthene of the hypersthene 

 rock, being surrounded with a deeper rind of hornblende. 

 This circumstance occurs in the diallage from Baste, in the 

 Hartz as described by Kohler. It is also observed, but 

 more characteristic, in the diallage of Gabbro, from the 

 village of Prese, between Bornio and Tirano in the Valte- 

 line. There the smaller portions consist wholly of horn- 

 blende, and the larger of diallage with a border of horn- 

 blende ; the latter is splendent and brown, and resembles 

 much the hypersthene in colour. It melts on charcoal 

 into a greenish-black bead. 



Pinchbeck mica, iron pyrites, and titaniate of iron occur 

 as accidental constitu tents, but in small quantity ; frequently 

 serpentine occurs in some varieties. The cross fracture of 

 diallage has great similarity with that of serpentine. 

 Hence, we are apt to overlook the coarse mixture of the 

 former, which distinguishes it from the latter. Serpentine 

 has been considered by some as fine grained gabbro,- 

 but chemical analysis overturns this supposition. Gabbro 

 is very frequently coarsely granular. In this case the 

 labradorite predominates, but from the manner in which 



• These characters are not the same as those given by Berzelius, in his work 

 on rhe Blow-pipe. Rose, therefore, supposes that ihe former has not examined 

 the right minerals, notwithstanding tli* observation, that the specimens were re- 

 ceived from Haiiy. 



