ARTICLE V. 
DESCRIPTION OF THE STRUCTURE OF THE FOSSIL WOOD FROM THE COLO- 
RADO DESERT. 
Br PROF. GEO. C. SCHAEFFER. 
‘There are two distinct specimens of this wood, one marked ‘‘ Pilot Knob; they are, how- 
ever, identical in structure when examined under the microscope. 
. A slight examination, under a low power, gives a very different idea of the character of these 
woods from that obtained by closer examination with high powers, comparing different parts of 
the same specimen, 
The peculiarity consists in the winding or contorted medullary rays which at times close upon 
each other, and again open, to form a wide space. This, however, is an accident, as will pre- 
sently be shown, and not directly dependent upon the normal structure. | 
Transverse section. — This view shows indistinct annual rings of considerable thickness, 53 
being found in five-tenths of an inch. Beside these annual rings, there are found, in some cases, 
subordinate rings, showing a partial interruption of growth during one season. The medullary 
rays are very much bent, being sometimes folded closely upon themselves. They are formed of 
three rows of cells in breadth, but these cells are not in the same horizontal plane; they are 
narrow and short, (radially ) 
The ducts are large, of one kind, generally single, and when double, but seldom compressed 
so that their adhering sides become flat, as is the case in many woods. The cavity of the ducis 
is filled with dark-brown matter, while the thick walls are lighter in color. Except at or near 
the lines dividing the annual rings, the ducts are not arranged in continuously circular lines, 
but are interrupted in their order. 
The wood cells are rarely to be seen except around the ducts, and where the structure is least 
altered they are found arranged in regular rows in both directions; in some cases with thick 
walls and narrow cavities, in others in thin walls and larger cavities. Where these cells are 
least altered they are also brown in color; the more they are changed the lighter they become ; 
and generally at a distance from the ducts they entirely disappear. The space between the 
medullary rays is then filled with a transparent siliceous paste, which in some places seems to 
show waving lines marking out the boundaries of successive deposits. It will now be easy to 
explain the cause of the folds in the medullary rays. The tree, before silification, had partially 
decayed, the cells of the rays, the ducts, and the surrounding wood resisted, or rather did not 
decay at all, and when pressure was made upon the whole mass the decayed portions gave way, 
and the rays bent, but as they did not quite touch each other, owing to the interposed ducts 
and unaltered wood, cavities were left in which the amorphous paste was deposited. 
The uniformity of this action, and the curious appearance caused by it, at first seems to 1 
cate an original difference in the durability of the different parts, but this is not probable. 
ndi- 
The 
