REPOET ON THE DEEP-SEA DEPOSITS. 
815 
continental rocks, and the difficulty increases still more when, far from the coasts, the 
terrigenous materials transported by icebergs and atmospheric currents are encountered. 
However, what permits us to pronounce upon the relative age of these particles, 
derived from showers of ashes or from the alteration of rocks of recent eruptions, is their 
position in the deposits ; they are distributed in all the pelagic deposits in process of 
formation. They are found in the superficial muddy layers forming the bottom of the 
sea at points where mechanical actions could not have torn them from the subsoil and 
transported them to a distance ; indeed, we must regard the bottom of the ocean as every- 
where covered by deposits made up of free particles which have accumulated during 
long ages upon the solid rocks constituting the true subsoil. It must then be admitted 
that the muddy layer into which the dredge or trawl can sink only a few feet from the 
surface is of recent age, or at the most Tertiary in certain points of the Pacific, where 
numerous vertebrate remains were procured. We have still another proof of the recent 
age of these volcanic ashes in the fact that the vitreous particles have in many instances 
stfil preserved their original fresh characters, whereas in the older geological formations 
similar particles have undergone profound alteration. The same argument holds good 
for the isolated mineral particles associated with the vitreous splinters. Although altera- 
tion has commenced in an appreciable manner in many of these minerals and fragments, 
it may be said that in the majority of cases it is not far advanced. 
In the year 1883 we published a memoir^ on the characters of volcanic ashes and the 
products of the disintegration of pumice and lapilli found in marine deposits. These 
characters have been in part given under the heading of pumice. In addition to the 
characters due to form there pointed out, the abundance of embryonic crystals and 
of skeletons of crystals may be regarded as likewise characteristic of these particles. 
The presence of crystals arrested in their development may easily be accounted for if we 
remember that the vitreous material which encloses them has been suddenly cooled, and 
that molecular changes have consequently been suddenly interrupted. 
The mineral particles in a deep-sea deposit having been derived from a great variety 
of sources, it is as a general rule impossible to say which of the volcanic particles have 
been derived from the basic, neutral, or acid series of rocks, and owing to this mixture 
chemical analysis is not available as a means of interpretation. Sometimes, however, it 
is possible to state with considerable certainty that the volcanic particles have been 
derived from a shower of ashes from a single eruption, as, for instance, in the case 
represented in PI. IV. fig. 3 from the South Pacific, Station 281, 2385 fathoms. Here 
the coarser particles have fallen upon a Red Clay, the point of junction being represented 
by the dark line in the centre of the section, finer and finer particles lying in layers 
above these just as they have fallen more slowly through the water. In PL XXL 
fig. 2 the larger mineral particles of this volcanic ash are shown at the line of junction 
* Proc. Boy. Soc. Edin., vol. xii, pp. 474 et seq. 
