68 ROCK-METAMORPHISM. 



86 and 94; but it seldom, if ever, occurs in this form. In 

 the annexed woodcut, fig. 1, the outside dotted lines represent a 

 cross section of the primary rectangular prism of peridote ; the 

 thick continuous lines within represent the secondary rhombic 

 modification ; the broken lines represent the cleavage, Usually 

 the prism has eight, ten, or twelve lateral faces ; a cross section, 

 being thus many-sided, has a somewhat rounded or rather ellip- 

 tical outline. Cross sections of peridote are figured in ZirkeVs 

 work with six sides'*. 



The cleavage of peridote consists of three dissimilar sets, two 

 being parallel with the prismatic faces of the rectangular pri- 

 mary, and one conformable with the basal faces ; consequently 

 their mutual intersections are at right angles to one another 

 (see inner broken lines in fig. 1), as may be observed in the 

 cleavage-solids which occasionally occur in masses of this 

 mineral common at Unkel and elsewhere. 



Turning to augite and hornblende, both belong to the mono- 

 clinic crystalline system ; their respective crystals, which often 

 occur as six- or eight-sided prisms, are usually less modified 

 than those of peridote; and moreover, on account of their 

 inclination, they are further differentiated from crystals of the 

 last mineral. The simplest modification of the primary of 

 hornblende, when it obliterates, as stated of peridote, all the 

 original faces, gives rise to an oblique rhombic prism, whose 

 acute and obtuse angles are respectively 55 30' and 124 30' 

 (see fig. 3) . The corresponding modification of augite yields 

 angles 87 5' and 92 55', as in fig. 2. It will thus be seen that 

 the cross section of a rhombic prism of augite differs extremely 

 little in its angular measurements from a similar section of 

 peridote, and that both are nearly a square. The decidedly 

 rhombic form of the cross section of hornblende need not be 

 mentioned in this comparison. 



The basal modifications in each of these three minerals render 

 it difficult to determine the form of their respective longitudinal 

 sections, depending, as the question does, as to whether such 

 section be a true one or not, on its parallelism with the vertical 

 crystalline axis. 



* Die mikroskopische Berschaffenheit der Mineralien und Gesteine,pp. 99, 

 216. None of Zirkel's figures represents a true cross section of peridote, as 

 they are too rhombic j they must be somewhat oblique to the vertical axis 

 instead of at a right angle. 



