168 THE VOYAGE OF H.M.S. CHALLENGER. 



replaced, are arranged. These facts, showing a phenomenon quite the reverse of an 

 uralitisation, are more common and also more distinct as the hornblende is more altered. 

 We may observe that small green crystals of augite also occur bordering sections of 

 olivine, but even although this is the case the olivine is not appreciably decomposed. 



Hornblende is represented in all the preparations of the volcanic rocks of Camiguin, 

 and is at once distinguished by its yellow-brown colour, which is sometimes rather 

 dark. Unlike augite, it is never found in the form of microliths, and it always 

 belongs to the first phase of consolidation. The sections rarely present a sharp 

 crystallographic outline ; they are always rounded and bordered with a black aureole of 

 magnetite interlaced with pale-green augite microliths. The crystals are often deeply 

 indented and broken, some portions lying at a little distance. The sections show in 

 some cases cleavages of about 124°, and hexagonal outlines corresponding to traces of 

 the prism and of the face cofoo, Sections parallel to the vertical axis are frequently 

 laminated and broken at the edges, thus acquiring a close resemblence to biotite. 

 Pleochroism is clearly marked, <y>/3>a being observed. This hornblende is often 

 twinned according to the ordinary law. It is unnecessary to discuss the alteration 

 into magnetite and the zone of augitic microliths, still the rock presents the finest 

 examples of this decomposition. It may be followed from one section bordered with 

 some grains of magnetite to another completely impregnated by this opaque oxide 

 or little crystals of almost colourless pyroxene. The hornblende is sometimes zonary, 

 and alteration has not taken place equally throughout the crystal. In such cases 

 a sort of frame of perfectly fresh hornblende may be observed surrounding an opaque 

 nucleus in which magnetite is accumulated. Sometimes large crystalloids of horn- 

 blende are joined, without the interposition of a matrix, to sections of plagioclase ; this 

 association, one might say this interpenetration, of the two minerals is common enough 

 to be worth pointing out. Sometimes small prisms of hornblende are enclosed in 

 felspathic sections, and the mineral also occurs associated with olivine. The last- 

 named mineral does not always occur in the rock ; when it appears it assumes the form 

 of sporadic grains, sometimes grouped in threes or fours, and frequently of considerable 

 size. Olivine does not exhibit crystallographic outlines, but it may be distinguished 

 at a glance from augite, as it is almost colourless or of a pale pink tinge, and from 

 felspar by the fissures which furrow its surface. Some lines of this network of fissures 

 are clearly denned and parallel; examination in convergent light shows them to be 

 arranged following the plane of the optical axes, the cleavage being thus parallel to the 

 pinacoid OP. This mineral is quite undecomposed, being perfectly colourless, except 

 at the edges of the sections, which assume a reddish tint, and it contains inclusions 

 of magnetite and bubbles of gas. 



Some comparatively rare but characteristic sections occurring in the rock should be 

 classed with bronzite. Although very small, they are easily distinguished from augite 



