of EdiThburgli, Session 1883-84. 
507 
which are neither of volcanic nor coral origin, the sediments are 
essentially composed of a mixture of sandy and amorphous matter, 
with a few remains of surface organisms, to which we give the name 
of muds, and which may be distinguished macroscopically by their 
colour. We distinguish them by the names, blue, red, and 
green muds. 
(2) Around volcanic islands the deposits are chiefly composed of 
mineral fragments derived from the decomposition of volcanic rocks. 
These, according to the size of the grains, are called volcanic muds 
or sands. 
(3) l!7ear coral islands and along shores fringed by coral reefs, 
the deposits are calcareous, derived chiefly from the disintegration 
of the neighbouring reefs, but they receive large additions from 
shells and skeletons of pelagic organisms, as well as from animals 
living at the bottom. These are named, according to circumstances, 
coral or coralline muds and sands. 
Let us now see what are the chief characteristics of each of these 
deposits. 
Blue mud is the most extensive deposit now forming around the 
great continents and continental islands, and in all enclosed or 
partially enclosed seas. It is characterised by a slaty colour which 
passes in most cases into a thin layer of a reddish colour at the 
upper surface. These deposits are coloured blue by organic matter 
in a state of decomposition, and frequently give off an odour of 
sulphuretted hydrogen. When dried, a blue mud is greyish in 
colour, and rarely or never has the plasticity and compactness of a 
true clay. It is finely granular, and occasionally contains fragments 
of rocks 2 centimetres in diameter ; generally, however, the minerals, 
which are derived from the continents, and are found mixed up with 
the muddy matter in these deposits, have a diameter of 0*5 mm. 
and less. Quartz particles, often rounded, play the principal part, 
next come mica, felspar, augite, hornblende, and all the mineral 
species which come from the disintegration of the neighbouring 
lands, or the lands traversed by rivers which enter the sea near 
the place where the specimens have been collected. These 
minerals make up the principal and characteristic portion of blue 
muds, sometimes forming 80 per cent, of the whole deposit. 
Glauconite, thongh generally present, is never abundant in blue 
