Lisbon — Pebbles on the Central Plateau of Brazil. 13 



ciently flat to form a solid base. Number 3 is the only one 

 of the collection which fails in this respect, showing an irregu- 

 lar and pointed base, seeming to have been partly polished. 

 Number 1 has a polished facet that may be considered plane 

 and which is opposite another facet also polished and perfectly 

 convex which is like the surface of any round, polished pebble. 



There are small pits in Nos. 1 and 2 although not very deep. 

 All of these pebbles have conchoidal fractures at the base of 

 the polished faces and on the edge of the base. Photograph 

 No. 3 shows these fractures clearly. The concave part of 

 these fractures is as perfectly polished as the surface of the two 

 facets. The two specimens 1 and 3 show this fracture in the 

 lower edge, resembling a broken lip. These breaks seem to be 

 produced by another body rubbing against the pebble after it 

 was already polished. 



The polished surfaces of these three samples have blackened 

 spots, formed by microscopic lichens, and the small pits are 

 sometimes filled with them. This vegetation is limited by a 

 line along the lower part of the pebble. Below this line and 

 over the whole base there is no sign of vegetation and the 

 stone is reddish, showing that it has been lying half buried in 

 the clay of the plateau for a long time, whilst the upper part 

 was exposed to the action of the atmosphere. In No. 3 it is 

 clear that part of the polished surfaces were under clay. This 

 specimen, is especially notable because its lower part is not 

 smoothed but is full of rounded pits which seem to have been 

 caused by rubbing against other hard rocks. 



(b) The three other samples of this type resemble in shape a 

 regular pyramid with a roundish base. They measure 6 or 7 

 centimeters in diameter at the base and about 4 in height. In 

 other respects they are like those already described. Like 

 them, they have a more or less plane base, the other faces 

 tending towards convexity and showing more or less rounded 

 edges. 



Number 4: shows four perfect facets and corresponding- 

 edges. A sample which was not photographed has more con- 

 vex facets and has a conical shape. The polished facets join 

 upon a rough base. 



Number 4 has near the base a slightly concave fracture with 

 polished edges which seem to show that it was there before the 

 polishing took place and, in spite of this, was not polished in 

 its concave part. 



2. Facetted ijebbles of different shapes.— Of these four sam- 

 ples were gathered. 



(c) Number 6 is flat in shape and has two equally developed 

 opposite faces, forming between them a very sharp angle. The 

 base is rough, full of small holes and sufficiently flat to be 



