648 OREGON AND NORTHERN CALIFORNIA. 
hills, which extend along for many miles, show an imperfectly co- 
lumnar structure. 
Besides the columnar structure, there is often a tendency to damina- 
tion in the basalt. ‘The lamine, in some places, are parallel with the 
bases of the columns, and arise from a concentric structure. In other 
instances, when the structure is not columnar, the rock splits into 
large slabs, sometimes less than an inch thick. ‘This structure may 
often be detected on a worn or exposed surface, when it is not apparent 
on a fresh fracture. ‘This lamination may be observed in the grayish- 
blue basalt, four or five miles above Vancouver. A still more perfect 
example of it was observed by the writer in a broad basaltic wall that 
crosses a ridge north of Elk Mountains. The lamine run lengthwise 
with the wall, parallel with the sides of the dike, and separate easily. 
Decomposition.— The compact basalts are firm and durable rocks. 
Decomposition takes place slowly, and produces a chocolate-coloured 
soil, sufficiently loose in its texture, and always fertile. The basaltic 
hills of the prairie region are usually covered with soil, except at top, 
where the rock generally outcrops. 
In a section of the soil near Elk River, there was one foot of dark- 
coloured soil, and four feet of deep-red earth below, resting upon a 
bluish-gray basalt, without cellules. All of this earth had proceeded, 
beyond doubt, from the decomposition of the basalt ; yet this rock was 
perfectly fresh and unaltered to within a sixteenth of an inch of the 
surface; and this exterior discoloured crust was nearly as hard and 
firm as the part below. I looked in vain for any intermediate step in 
the process of decomposition between the red earth above and the 
grayish-white or yellowish surface of the rock. It appears that when 
decomposition proceeds beyond this discoloration, the altered rock 
separates at once in scales or grains, which unite with the earth (or 
bed of decomposed basalt) that lies above. ‘The iron of the rock, 
visible in minute grains, is set free, and probably promotes the decom- 
position; changing to a red oxyd it gives the red colour to the earth. 
Large rounded masses of superficially discoloured basalt lie imbedded 
in the lower part of the basaltic earth, which appear to have been 
separated from one another by this process of decomposition. In this 
manner a concentric structure is indicated which otherwise we should 
not have suspected in this compact rock. ‘These facts are similar to 
those observed in New South Wales. (p. 513.) 
Basaltic Conglomerates. — Conglomerates, tufas, and breccias of 
basaltic or volcanic origin, occur with the basaltic rocks and lavas in 
