.20 



NATURE 



[Fed. 8, 1877 



position, while others are little altered. Some are coated with 

 the peroxide of manganese, or have streaks of this substance 

 running through them. They are the most frequent nucleus of 

 the manganese nodules, to which I shall presently refer. Some 

 specimens which were dredged from a depth of over three miles, 

 ■will, when dried, float for weeks in a basin of water; others, 

 which have undergone partial decomposition, sink at once. 



They present a great variety of texture and composition. They 

 are white, grey, green, or black in colour. They are highly 

 vesicular, or rather compact and fibrous. There would appear 

 to be every gradation fro.Ti common feldspathic to dark green 

 pyroxenic kinds. 



We find in them crystals of sanidin, auglte, hornblende, 

 olivine, quartz, Incite, magnetite, and titaniferous iron. Mag- 

 netic iron ore was found in all the specimens examined, either in 

 crystals or in the form of dust. The other minerals vary in kind 

 and abundance in the diiferent specimens. The same crystals 

 which we find, in the pumice occur in all the kinds of ocean 

 deposits. 



Sources of the Pumice-Stones. 



The pumice-stones which we find at the bottom of the sea 

 have most likely all been formed in the air. Some of thetn may 

 have fallen upon the sea ; but the great majority seem to have 

 fallen on land, and been subsequently washed and floated out to 

 sea by rains and rivers. After floating about for a longer or 

 shorter time they have become water-logged and have sunk to 

 the bottom. Both in the North Atlantic and Pacific small 

 pieces of pumice were several times taken on the surface of the 

 ocean by means of the tow-net. Over the surface of some of 

 these serpu'se and algae were growing, and crystals of sanidin 

 projected, or were imbedded in the feldspar. During our visit 

 to Ascension there was a very heavy fall of rain, such as had not 

 been experienced by the inhabitants for many years. For several 

 days after many pieces of scorite, cindei-s, and the like were 

 noticed floating about on the surface of the sea near the island. 

 Such fiagments may be transported to great distances by 

 currents. 



On the shores of Bermuda, where the rock is composed of 

 blown calcareous sand, we picked up fragments of travelled vol- 

 canic rocks. The same observation was made by General 

 Nelson at the Bahamas. Mr. Darwin noticed pieces of pumice 

 on tbe shore of Patagonia, and Prof. L. Agassiz and his com- 

 panions noticed them on the reefs of Brazil. During a recent 

 eruption in Iceland, the ferry of a river is said to have been 

 blocked for several dajs by the large quantity of pumice floating 

 down the river and out to sea. All the pumice which we find 

 need not be of quite recent origin. Mr. Bates informs me that 

 great quantities of pumice are continually being floated down 

 the Amazon. These come from near the foot of the Andes, 

 where the head-waters cut their way through fields of pumice- 

 stones. In the province of Wellington, New Zealand, two of 

 the rivers run through areas covered with pumice, and during 

 floods bear great quantities out to sea. 



Prof. Alex. Agassiz has kindly furnished me with the following 

 note : — 



" The river Chile, which flows through Arequipa, Peru, has 

 cut its way for some thirty miles through the extensive deposits 

 of volcanic ashes which form the base of the extinct volcano, 

 Misti. Some of the gorges are even 500 feet in depth, forming 

 regular caiions. The whole lengih of the river bottom is covered 

 by well- rolled pieces of pumice from the size of a walnut to that 

 oi a man's head. In the dry season (winter) there is but little 

 water flowing, but in the summer, or rainy season, the river, 

 which has a very considerable fall (7,000 feet in a distance of 

 about ninety miles), drives down annually a large mass of these 

 rolled pumice-stones to the Pacific. The volcanic ashes are not 

 recent. There is no tradition among the Indians of any eruption 

 within historic times." 



Capt. Evans, the present hydrographer to the navy, informs 

 me that he frequently picked up pumice on the Great Barrier 

 Retf of Australia. 



Vo'canic Ashes. 



Near volcanic centres, and sometimes at great distances from 

 land, we find much volcanic matter in a very fine state of division 

 at the bottom of the sea. This consists of minute particles of 

 feldspar, hornblende, augite, olivine, magnetite, and other vol- 

 canic minerals. In the South Pacific, many hundred miles from 

 land, and from a deptfi of 2,300 fathoms, the trawl brought up 

 a number of pieces of tufa entirely composed of these com- 

 minuted fiagments. These particles appear to me to have been 



carried to the areas, where we find them, by winds, in the form 

 of what is known as volcanic dust or ashes. Sir Rawson 

 Rawson sent to Sir Wyville Thomson a packet of the volcanic 

 ashes which fell on the island of Barbadoes, after an eruption in 

 1812 on the island of St. Vincent, W.I., one hundred and sixty 

 miles distant. I have examined this, and find it made up of 

 fragments of the same character as those in the tufa to which I 

 have just referred, some of the particles being perhaps a lit'le 

 larger. We have sometimes found this ash in considerable 

 abundance mixed up with the shells in a globigerina ooze. In 

 the deposits for hundreds of miles about the Sandwich Islands 

 there are many fragments of pyroxenic lava, which I believe 

 have been borne by the winds, either as ashes, or in the form of 

 Pele's hair. 



At Honolulu we were informed that threads of Pele's hair 

 were picked up in the gardens there after an eruption of Kilauea, 

 one hundred and eighty miles from the volcano. This Pele's 

 hair bears along with it small crystals of olivine. 



Obsidian and Lava Fragments. 



Small pieces of obsidian and of feldspathic and basaltic lavas 

 were frequently found in deposits near volcanic islands. 



At two stations in the South Pacific, many hundred miles 

 from land, we dredged pieces of this nature of considerable size 

 larger than ordinary marbles. It is difficult to account for the 

 transference of these fragments to the places where they were 

 found. It is, however, in this region, and this alone, that it 

 may be necessary to bring in a submarine eruption to account for 

 the condition of things at the bottom. 



A consideration of these observations, and the specimens 

 which are laid on the table, will, I think, justify the conclusion 

 that volcanic materials, either in the form of pumice-stones, 

 ashes, or other fragments, are universally distributed in ocean 

 deposits. 



They have been found abundantly or otherwise in our dredg- 

 ings, according as these have been near or far from volcanoes, 

 or as there has been much or little river and coast detritus, or 

 few or many remains of surface organisms in the deposits. 



Some of the Products of the Decomposition of Volcanic Debris. 



Clay. — Pure clay, as is well known, is a product of the de- 

 composition of feldspar, and the clay which we find in ocean 

 deposits appears to have had a similar origin. 



In the deposits far from land the greater part of the clay origi- 

 nates, I believe, from the decomposition of the feldspar of frag- 

 mental volcanic material, which we have seen to be so universally 

 distributed. 



Pumice-stone is largely made up of feldspar, and from its 

 areolar structure is pecaliarly liable to decomposition. Being 

 permeated by sea-water holding carbonic acid in solution, a part 

 of the silica and the alkalies are carried away, water is taken up, 

 and a hydrated silicate of alumina or clay results. 



Like most clays our ocean clays contain many impurities, 

 these last being as varied as the sources whence the materials of 

 the deposits are derived. 



Let us briefly enumerate the sources of these materials. 



We have (i) the matters derived from the wear of coasts, 

 and those brought to the sea by rivers, either in a state of sus- 

 pension or solution. The material in suspension appears to be 

 almost entirely deposited within two hundred miles of the land. 



Where great rivers enter the sea, and where we have strong 

 currents, as in the North Atlantic, some of the fine detritus may 

 be carried to a greater distance, but its amount can never be 

 very large. In oceans affected with floating ice we have land 

 debris carried to a greater distance than above stated ; for in- 

 stance, we can detect such materials in the deposits of tiie North 

 Atlantic as far south as the 40th paral el N,, and in the South 

 Pacific as far north as the 40th parallel S. 



Some of the substances in solution, as carbonate of lime and 

 silica, are extracted by animals and plants to form their shells 

 and skeletons ; these last, falling to the bottom, form a globige- 

 rina, a pteropod, a radiolarian, or a diatom ooze. We have also 

 the bones of mammals and fish mixed up in different kinds of 

 deposits. These, as well as animal and vegetable tissues, gene- 

 rally are a source of phosphates, fluorides, some oxides of iron, 

 and possibly of other inorganic material. 



Sir Wyville Thomson, early in the cruise, suggested that much 

 of the inorganic material in deposits is derived from the source 

 to which I have just alluded. Our subsequent observations 

 have, I think, shown that originally Sir Wyville gave too much 



