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APPENDIX. 
TRES MARIAS AND SAN BLAS. 
WEST COAST OF MEXICO. 
The Ties Marias, situated 1° 15" west of San Bias, consist of three large islands, 
steep and rocky, to the westward, and sloping to the eastward with long sandy spits. Off 
the S. E. extremity of Prince George’s Island (the centre of the group) we found that the 
soundings decreased rapidly from seventy-five fathoms to seventeen, and that after that depth 
they were more regular. Two miles from the shore we found ten and twelve fathoms, bad 
holding ground. There is nothing to make it desirable for a vessel to anchor at these islands. 
Upon Prince George’s Island there is said to be water of a bad description ; but the landing 
is in general very hazardous. 
There are passages between each of these islands. The northern channel requires no 
particular directions: that to the southward of Prince George’s Island is the widest and best; 
but care must be taken of a reef lying one third of a mile off its S. W. point, and of a shoal 
extending a mile and a half off its south-eastern extremity. 1 did not stand elose to the 
south Maria, but could perceive that there were breakers extending full three quarters of 
a mile off its S. E. extremity; and I was informed at San Bias that some reefs also extended 
from two to four miles off its south-western point. There is an islet off the north-west part 
of this island, apparently bold on all sides ; but I cannot say how closely it may be ap- 
proached. 
From the south channel Piedro de Mer bears N. 76° E., true, about forty-five miles. 
It is advisable to steer to windward of this course, in order that, as the winds, during the 
period at which it is proper to frequent this coast, blow from the northward, the ship may be 
well to windward. 
The Piedro de Mer is a white rock, about a hundred and thirty feet high, and a hundred 
and forty yards in length, with twelve fathoms all round it, and bears from Mount St. Juan 
N. 77° W. thirty miles. 
Having made Piedro de Mei', pass close to the southward of it, and unless the weather 
is thick, you will see a similarly shaped rock, named Piedro de Tierra, for which you should 
steer, taking care not to go to the northward of a line of bearing between the two, as there 
is a shoal which stretches to the southward from the mainland. This course will be 
S. 79° E. true, and the distance between the two rocks is very nearly ten miles. 
To bring up in the road of San Bias, round the Piedro de Tierra, at a cable’s length 
distance, and anchor in five fathoms, -with the low rocky point of the harbour bearing N. i E., 
and the two Piedros in one. This road is very much exposed to winds from S. S. W. to 
N. N. W., and ships should always be prejiared for sea, unless it be in the months in which 
the northerly winds are settled. Should the wind veer to the westward, and a gale from that 
quarter be apprehended, no time should be lost in slipping and endeavouring to get an 
offing, as a vessel at anchor is deeply embayed, and the holding ground is very bad. In case 
of necessity a vessel may cast to the westward and stand between the Piedro de Tierra and 
the Fort Bluff, in order to make a tack to the westward of the rock, after which it will not 
be necessary again to stand to the northward of a line connecting the two Piedros. 
The road of San Bias should not be frequented between the months of May and 
December, as during that period the coast is visited by storms from the southward and 
