320 
THE VOYAGE OF H.M.S. CHALLENGER. 
rocks. These two micas are very characteristic of terrestrial rocks and mineral particles. 
Olivine, distinction difficult, but sometimes irregularly-bounded fragments, decomposing 
into serpentine, and with fragments of older eruptive rocks. Pyroxene (a) Rhombic, 
Bronzite, lamellar aggregates, generally large fragments found with older eruptive 
rock debris, with peridotite fragments. (6) Monoclinic, Augite, fragments irregularly 
bounded or bounded by cleavage planes, transforming into uralite or chlorite, rarely 
vitreous inclusions, associated with fragments of diabase. Diallage, grains bounded 
by cleavage planes, associated with mineral particles and fragments of older eruptive 
rocks. Quartz, grains generally without crystallographic outlines, rounded or angular, 
sometimes covered with oxide of iron, liquid inclusions, some with carbonic acid or 
small cubic crystals, needles of rutile, tourmaline, scales of chlorite, hematite, &c. 
Occurs always with granitic, porphyritic, schisto-crystalline rocks, or with fragments of 
continental sedimentary rocks ; the minerals and rocks associated with the quartz 
grains give a clue as to the matrix rock. In some cases grains quite rounded, and 
all of about the same dimensions, with thin coating of limonite, found far from coasts in 
pelagic deposits, are to be considered as wind-borne.^ Rutile, small grains, or microscopic 
prismatic crystals imbedded in schistose rock particles, always associated with continental 
debris. Serpentine, compact or fibrous grains, associated with fragments of older 
crystalline rocks, principally with peridotic rocks. Tourmaline, often in small prismatic 
fragments of crystals, almost always of continental origin and associated with debris of 
crystalline schists, granitic rocks, &c.^ Zircon, small quadratic crystals, more or less 
rounded, as in the case of tourmaline, almost always of continental origin, and found 
with debris of crystalline schists and of older eruptive rocks ; associated frequently 
with quartz grains, and other minerals derived from the disintegration of sedimentary 
rocks.^ 
The above are the principal mineral particles in the marine sediments to which we 
attribute a continental origin. The mineral characters of many of them are not, how- 
ever, of a nature to give certain and satisfactory indications ; especially is this the case 
for the particles of apatite, chlorite, chromite, epidote, garnet, hematite, magnetite, 
olivine, and pyrites. It is only the geographical position, along with the mineralogical 
associations, that permits a satisfactory determination in any particular case. On the 
other hand, for several of the species a continental origin seems to be indicated beyond 
all doubt ; this is the case with glaucophane, white mica, sericite, tourmaline, zircon, 
microcline, and for the great majority of the grains of quartz. 
' See Plate XXV^I. fig. 3; these rounded grains of quartz are here a.ssociated with particles of felspar, green horn- 
blende, glassy volcanic fnigmcnts, grains of manganese, very rarely fragments or particles of vein quartz, milky, and of 
irregular form, found with continental land debris. 
* See Plate XXVII. fig. 6; black fragments of prismatic crystals of tourmaline, with rounded grains of quartz 
glauiymite, and zircon. 
* See Plate XXVII. fig. 4 ; small bipyramiilal crystals, one in the centre, the other a little higher in the figure. 
