Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.2): between the years 1826 and 1836 : describing their examination of the southern shores of South America, and the Beagles's circumnavigation of the globe

TOSCA GRANITE — SOUNDINGS. even a foot, in the height of the water, makes a change of cables'lengths, if not of miles, in the position of the limiting line between water and land. In consequence it is very dangerous for ships to approach that shore ; and, although the bottom is in many places soft, often extremely deep mud, there are other spaces in which hard lumps of tosca* are found, almost as injurious to a ship's bottom as actual rock. I am not aware that there is any granite on the south side of the river Plata, near the shore ; and although the name ' Piedras"' would incline one to suppose there are rocks or stones near it, I could only jfind tosca. But towards the northern shore rocks are found, and the dangerous shoal called ' Banco Yngles,' is said to have a granitic foundation. The ridge, of which the English Bank is the north-west extremit}"^, extends eastward, inclining to the south, considerably beyond a line drawn from Cape Santa Maria to Cape San Antonio, and less than ten fathoms water may be found upon it out of sight of land. Northward of the ridge the deptli of water varies from ten to thirty fathoms over a very soft bottom of bluish mud ; and to the southward of it there are from twelve to three fathoms (diminishing as you approach San Antonio) over a softish bottom of brown or yellow muddy sand.f When it is considered that three very large rivers, besides a host of smaller streams, enter the ocean by the estuary whose more remarkable features we are noticing, that two of those rivers are flooded periodically by tropical rains,| and that very heavy gales assist in emptying or filling the shallow wide gulf, in which floods of fresh water contend against the volume of a powerful ocean ; not only will frequent * Tosca is a kind of hardened earth, rather than soft stone, about the consistence of slightly baked clay : it is of a dark brown colour, and varies in hardness from that which is almost stony, to the texture of a sound old cheese. t Near Cape San Antonio and Point Tuyu there is very soft mud. I The Paraguay rises so far northward, that (excepting a portage of three miles) a canoe may go from Monte Video to the mouth of the Amazon.