THE TRAP DYKES OF THE ORKNEYS. 881 



usual variety. In either case the result is the completion of the crystalline form, which is 

 that of the external mineral, and is often perfectly sharp and well preserved. But more 

 usually this zone is composed of both minerals in parallel growth, each forming part of the 

 external rim, and lending its form to the part it constitutes. The augite often forms the 

 lateral faces, the hornblende the terminal ones (PI. I J. fig. 2). But their relative position 

 and amount follow no fixed rule. At one end there may be augite, at another hornblende, 

 or hornblende may be the earlier and form the inner part, while a mere film of augite 

 surrounds it, thickest at the corners over the prism faces and thinnest on the pinakoids, 

 and giving the whole the octagonal transverse section of pyroxene. Between crossed 

 nicols it is easily seen that the marginal augite is in optical continuity with the 

 scattered patches in the graphic augite-hornblende intergrowth in the zone within ; and 

 that it is in parallel growth with the hornblende is shown by the disposition of the 

 cleavages, and the simultaneous extinction of both minerals in certain sections, those, 

 namely, which are parallel to the ortho-diagonal. The pinakoidal faces of both minerals 

 are parallel to one another. The relation of their terminal planes could not be made out, 

 owing to the fact that where these crystalline faces of one mineral are perfect, those of 

 the other are deficient. The crystals are frequently twinned, and the twin plane passes 

 straight through both minerals (see PI. II. figs. 1, 2, 3, 4, 5, and description, p. 904). 



This is the commonest, and at the same time the most highly developed and com- 

 plex type of structure, but many sections are found in which all these zones are not 

 present. The centre may be an augite-hornblende intergrowth ; the periphery ordinary 

 hornblende or augite ; or on a nucleus of greenish-brown laminated hornblende, a border 

 of ordinary hornblende may have formed. There is, in fact, a very great variety of 

 combinations. But the order of succession is invariably that given, and is never 

 reversed, although certain of the zones may be wanting. The dark brown hornblende 

 full of minute magnetite dust is never any but the innermost zone, and never occurs- 

 without another zone of whatever sort surrounding it. Neither does the graphic inter- 

 growth of hornblende and augite form the external surface in any case. The absence- 

 of certain elements may perhaps be explained as due to the section having missed that 

 part of the crystal which they constitute, and this is most probable when the central 

 zones are apparently absent. It is obvious that these structures are the result of a 

 complex series of periods of alternate crystal growth and destruction by corrosion. 

 Whether the augite-hornblende intergrowth is a direct formation or a resorption product 

 it is not easy to establish. It has rarely external crystalline outlines, and it is filled 

 with magnetite sometimes to such an extent as to be almost opaque, and in these respects 

 it recalls many instances of the complete or partial destruction of hornblende and substitu- 

 tion of augite and magnetite. But this does not explain the graphic structure, and it 

 seems more probable that this zone has originated by direct crystallisation from the 

 magma. Its external outlines may be those characteristic of augite or of hornblende, 

 but more usually are rounded or irregular. 



In the west dyke of Hoxa these complex phenocrysts are largest and most numerous. 



VOL. XXXIX. PART IV. fao. 33). 6 U 



