When we arrived at its base we found the stream bubbling among the fallen blocks.

For the next twenty-eight miles the river-course was encumbered with these basaltic masses.

Above that limit immense fragments of primitive rocks, derived from the surrounding boulder-formation, were equally numerous.

None of the fragments of any considerable size had been washed more than three or four miles down the river below their parent-source: considering the singular rapidity of the great body of water in the Santa Cruz, and that no still reaches occur in any part, this example is a most striking one, of the inefficiency of rivers in transporting even moderately-sized fragments.
The basalt is only lava which has flowed beneath the sea; but the eruptions must have been on the grandest scale.

At the point where we first met this formation it was 120 feet in thickness; following up the river-course, the surface imperceptibly rose and the mass became thicker, so that at forty miles above the first station it was 320 feet thick.