98 
THE VOYAGE OF H.M.S. CHALLENGER 
Lous Aoamiz 
OX THE PoUBTAJLta 
Plateau. 
death of ! ’rofc.-sor Bailey the examination of tlie deposits devolved upon Pourtales. In 
the deposits from off the American Coast from Cape Cod to Florida he found two well- 
marked varieties : 1 siliceous and calcareous. The siliceous deposits stretch along the 
coast as far as Cape Florida. The calcareous deposits are divided into Coral and Fora- 
miniferous - formations, and are found at the greatest depths, at the southern point of 
Florida, and off Cuba and the Bahamas. Pourtales remarked the coincidence between 
the limits of the siliceous deposits and the course of the cold current, and of the calcareous 
deposits and the warm current. He also distinguished a muddy deposit of much less 
extent, which he considered quite subordinate ; it was observed off the eastern part 
of Long. Block, and Martha Islands. He considered this deposit as related to the 
Tertiary formations, some traces of which appear- on Gay Head Reefs, Martha’s Vine- 
yard, and other localities in Massachusetts. The deposits in shallow water off the 
American coast are sandy, principally quartz grains, with a few grains of hornblende, 
felspar, and sometimes glauconitic, but the grains of glauconite may be derived from the 
disintegration of the geological strata. 3 In these sandy deposits he observes that the 
Fnraiiiinifera are distributed in zones, sometimes overlapping : thus the zone nearest the 
shore from 10 to 20 fathoms is relatively poor, containing only a few small Polystomelke ; 
then Miliolina is met with in small numbers at about 40 fathoms ; Truncatulina 
ad vena is found from 25 to 70 fathoms; Marginulina and Cristellaria begin at about 
3-5 fathoms and extend down to over 1000 fathoms. From a depth of 60 fathoms the 
sand becomes mixed with Globigerinse, which increase to such an extent that at a depth 
of 100 fathoms the shells are as abundant as the sand grains, marking the commencement 
of the calcareous deposit. 
In connection with the laying of a telegraph cable between Cuba and Florida, explora- 
tions werr begun in 1867, and continued for two years, occasionally under the personal 
supervision of L. Agassiz. The coral reef was found to be closely confined to the 
C'>ast of Florida, for the large reef-building Corals only acquire their full development 
near the surface, nut extending below 10 fathoms. The fauna inhabiting the reef con- 
-i-ts of u large number of animals of all classes, and is totally different from that of the 
de<p - a. From the reef the bottom is muddy down to 50 or 60 fathoms, and is covered 
with dead Mollusca and triturated fragments of Corals, with few living animals. Then 
follows a rocky plateau, which Agassiz calls the Pourtales Plateau, down to 100 and 
occasionally 200 fathoms, the bottom being a calcareous conglomerate with Molluscs and 
Coral-. The Foraminifcrous deposit is found in the Florida Strait at moderate depths, 
at point- where the rocky bottom is hidden. Pourtales observes that this calcareous 
1 Report of Superintendent of U.S. Coast Survey for 1869, pp. 220-226, Washington, 1872. 
Pourtat 1 1 in that this Globigerina Ooze, one of the most important of oceanic deposits, was first observed in 
1“ >3 by Li .'• > njt - Cru-. en and Muffin during their investigation of the Gulf Stream in connection w ith the U.S. Coast 
Sarrey ; it w n »u •• <juently noticed during the preliminary survey of the route for the Transatlantic cable. 
1 From the Greensand of New Jersey (Pourtales in Rep. U.S. Coast Survey for 1869). 
