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



more closely, one sees that this appearance is due to the presence of tufts, filaments, 

 and lamellae intertwined in all directions. Their aspect and their polarisation 

 tint recall precisely the appearance of certain zeolites. The nepheline has been 

 but slightly subjected either to the corrosive action of the magma or to mechanical 

 deformations. It is this that distinguishes it at a glance from the sanidine, with 

 which, but for the twins, and the peculiarities that are to be described in the latter, it 

 might perhaps be confounded. The relief of the contours and the phenomena of 

 polarisation, so far as colour is concerned, give us little help in differentiating these 

 two minerals at first sight. But they can be distinguished by the irregular breaks of 

 the sanidine, the elongation of its sections, and by its Carlsbad twins. 



The sanidine occurs, as we have just indicated, in lamellae elongated parallel to 

 the edge PjM; indeed it can be observed that certain sections, in which the Carlsbad 

 twinning does not appear, and which are therefore almost parallel to M, have elongated 

 rhomboidal forms, in which are seen the outlines of the faces of the prism and of P or x. 

 This mineral is almost always cracked by more or less irregular fissures, which seem in 

 sections parallel to P to be perpendicular to the greatest length, while in those taken 

 parallel to M they are sensibly parallel to the vertical axis. The action of the magma 

 has often been exerted along these fissures, which are filled by the ground-mass ; this 

 action is also shown by the corrosion of the outlines of the mineral, so much so that no 

 rectilinear outlines are now to be found ; they are scooped out more or less deeply, 

 serrated and sometimes rounded off. Besides these corrosions there are other pheno- 

 mena in the sanidine that are to be attributed to the fluidity of the magma : the 

 sections appear dislocated and twisted ; the various fragments of a crystal are scattered 

 and overlap one another, and it is rare to see a section of sanidine in which one cannot 

 make out displacements and ruptures. In parallel light sections showing the composi- 

 tion plane of the Carlsbad twin distinctly exhibit straight extinction, which occurs 

 parallel to their greatest length. In convergent light sections cut perpendicularly to 

 the prismatic zone show in each of the two individuals one of the two axes situated 

 along the length of the section at opposite sides of the plane of vision. This seems to 

 indicate that in this sanidine the optical axes are in the plane of symmetry. 



Augite is one of the most widely distributed minerals in this rock. It occurs 

 firstly in large individuals, then microporphyritically, and lastly as immense numbers 

 of microliths in the ground-mass. We have here to describe the large augites of first 

 generation. The character which at once distinguishes them is their green tint; 

 these sections are very dichroic. The forms usual in this augite are octahedral 

 sections, with the sides that represent the traces of the prisms more developed than 

 those corresponding to the traces of the pinacoids. The prismatic cleavage is not 

 well marked, — on the contrary, lines of cleavage more distinct than these are to 

 be seen running parallel to the pinacoid ooPoo. Sections of the vertical zone are 



