BASALTIC AND IGNEOUS ROCKS. 647 
often constitute a series of layers. Along the Columbia and Wil- 
lammet the layers are from fifteen to fifty feet thick: they average 
thirty-five feet. Such beds piled upon one another form the palisades 
of the Columbia,—bluff walls two hundred feet high. The layers 
are very distinctly separated, and often cavernous recesses intervene; 
they frequently project in shelves, and the banks then appear ter- 
raced with vegetation. Up the Willammet, below the Falls, the east 
bank consists, for some distance, of the edge of a single layer, fifteen 
to twenty feet thick. Ascending farther, another layer is seen, and at 
the Falls a third is added. Others may be distinguished in the stcep 
declivities forming the high banks of the river above the Falls. The 
rock is often extremely ragged and cellular at the junction of two 
layers, although elsewhere compact. 
This occurrence in layers is the prevailing character of the rock 
high up the Columbia; and in some places, as at the Dalles, there is 
an alternation of the basalt with basaltic conglomerate or tufa. On 
the Snake River, similar basaltic layers have been described by Mr. 
Parker, as forming the steep walls. These walls are three hundred 
feet high, and in some places six to eight feet of conglomerate inter- 
vene between the solid layers. 
Since each layer is evidence of a distinct flow of melted basalt, we 
may gather from these facts some idea of the vast inundations of liquid 
rock which have covered this country. 
Some traces of a columnar structure may generally be detected in 
the basaltic rocks along the Columbia. Half a mile below Lower 
Cape Horn there is a natural wharf extending out into the river, which 
Lieutenant Walker described to me as consisting of hexagonal 
columns neatly fitted together; and there are also isolated columns, 
which are called the Ten-pins. Below Wallawalla, towards the Grand 
Rapids, there are fine exhibitions of basaltic columns. One peak, com- 
posed of these columns, looks like a square tower upon a conical base, 
and is called “The Windmill.” Other table summits, with bluff 
fronts, exhibit basaltic prisms in fine perfection. A still more re- 
markable example of basaltic architecture occurs on the Snake River. 
As there are several layers of basalt, so there are several successive 
ranges of columns, differing a little in size and perfection, yet all 
remarkable for their regularity. 
On the Willammet, below the Falls, there are imperfect prisms, 
eight feet in diameter, in a bed of basalt but twenty feet thick. In the 
vicinity of the Boundary Range, (lat. 42°,) the bluff fronts of basaltic 
