PHTHANITK. 107 



little opal. Mixed with the silica is ferric oxide or ferric hydrate, some- 

 times in very uniform distribution and again in patches or streaks. The 

 iron oxide is somewhat translucent and is soluble in dilute chlorhydric acid 

 with a vellow color. The mass is ordinarily intersected by extremely nu- 

 merous quartz veins. These are seldom more than 0.5""" in thickness, while 

 the thinnest are often visible under the microscope as mere white lines of 

 insensible width. The veins are usually continuous across the slide, but 

 may sometimes be ol)served pinching. When this is the case the fissure 

 narrows very gradually as the point is approached, as is the case with 

 fissures of all sizes in all rocks. There are nearly always, if not invaria- 

 bly, two sets of fissures in the phthanites, crossing each other at a high 

 angle, and all the phenomena are precisely similar to those accompanying 

 torsional fracture as investigated by Mr. Daubrce.' Small dislocations of 

 coin-se inevitably accompany fractures of this description. 



The veins are principally filled with crystalline quartz in which fluid 

 inclusions have not been detected with certainty. Besides the quartz, how- 

 ever, there are often found numerous prisms of zoisite. In the larger veins 

 these are usually arranged along the walls, from wliich they appear to have 

 grown. In the smaller veins long, jointed prisms, such as are illustrated in 

 Fig. 1 (page 78), often lie parallel to the strike of tlie vein. The zoisite in 

 these rocks is thoroughly characteristic, showing jointing of the crystals, 

 fluted surfaces, tolerably high refraction, yellow interference colors, and 

 extinction strictly parallel to the main axis, as given under the description 

 of the mineral. The prisms are too much fluted to give good cross-sec- 

 tions. In its mode of occurrence it differs somewhat from the zoisite of 

 the sandstones, for, while in the latter it is usually embedded in products of 

 metasomatosis, in the phthanites it occurs in an infiltrated mass and appears 

 to result from a reaction between the silica and the shale. In certain spots 

 the silica seems to have eaten into the walls of the veins, and here zoisite 

 is especially abundant. Sometimes, on the other hand, the veins show no 



'Mr. Daubrde has written sevcr.il papers on fractures in rocks, whicU are illustrated by experi- 

 ments They will be found in the Bull. Soc. g^ologique France, 1878-1879, 1881-1882, and in the Anuu- 

 aire du club alpin fran^ais, 1881, 1882. The figures of the second plate la the first paper in the latter 

 journal, which represent the fissures produced by torsion in glass plates, might, so far as I could tell, 

 be from photographs of somewhat weathered phthanites from the Coaet Ranges. 



