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



two individuals are joined in the plane of the rhombic section ; as the figure shows, this 

 plane is visibly inclined towards the face P' in the same direction as the extinction, 

 which is negative and of 39° for one individual, for the other 20°. 



The principal characters of the plagioclase in the rocks of Camiguin may be sum- 

 marised as follows. The optical properties of this mineral, its structure, groupings, and 

 twinnings, indicate that it represents a plagioclastic mixture intermediate between 

 oligoclase and labradorite. One of the most interesting features of this felspar is, that 

 this mineral has crystallised in these rocks with numerous and very well-developed 

 faces ; the traces of M P T I x y can be seen in the sections. This abundance of 

 faces is a somewhat rare occurrence, and one to be noticed. The zonary structure 

 is no less remarkable ; it manifests itself in all the sections, one may say. For the 

 external and internal zones there are found extinctions of very different values — which 

 point to variations in the chemical composition of the magma at various stages in the 

 growth of the mineral in question. Generally speaking, the extinctions for the internal 

 zones occur at less angles than for the external. We have therefore to admit that 

 the acidity of the magma has been decreasing in proportion as the felspar has gone on 

 developing. In certain cases the various layers of which the crystal is formed have 

 extinctions whose values gradually rise from the central zones to the periphery ; in 

 these cases the section presents undulating extinction. This zonary structure is, 

 moreover, characteristic of the intermediate felspars— oligoclase, labradorite, and above 

 all of andesine. No less conspicuous are the twins and groups of which our figures 

 furnish some examples. These plagioclastic sections are almost always striated, follow- 

 ing the albite law ; often the hemitropic lamellae are very thin, and appear as simple 

 lines. Twins following the pericline and albite laws are often seen in the same section, 

 sometimes that of albite only. In this latter case the plane of union between the two 

 individuals often appears indistinct. The form of the sections is very variable ; some 

 are seen to be symmetrical, with two opposite angles blunted ; they are more or less 

 parallel to the face P ; the more or less rounded lines are the traces of I and of T. 

 The sections with asymmetrical contours are generally cut in a plane very nearly 

 parallel to M ; nevertheless, thanks to the crystalline faces of this plagioclase, we 

 sometimes also notice sections that are parallel to M, and have a symmetrical appear- 

 ance. They can always be distinguished from the first (sections parallel to P), because 

 the cleavages are not equal, nor are they equally inclined to one another, as is the case 

 with prismatic cleavage. Moreover, the trace of a face may be observed, which 

 makes with one side, alternate or adjacent, an angle approaching a right angle. 

 The face h not being known, one may say, in the felspar in question, the conclusion 

 ought to be that we are here dealing with y, which again proves that the crystal has 

 been cut in a direction coinciding with M, or approaching that plane. The felspathic 



