Page:Popular Science Monthly Volume 69.djvu/297

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will be naturally renewed. A faulted rock bed will be cemented in the course of ages of pressure and of cementation.

The most interesting of these breaks in California is that recorded as the Portolá-Tomales fault. Its course can be plainly traced on the relief map. It enters the shore near the mouth of Alder Creek and near the low headland called Point Arena, in Mendocino County on the north, and runs to Chittenden, on the Pájaro River, in Monterey County, on the south. The line is almost perfectly straight, and its course and direction can be determined by placing a ruler on the map, using the line of Tomales Bay as an axis. This long, narrow, straight inlet is a resultant of past earthquakes, probably beginning in Tertiary times. It is bounded on the west by mountains which have their origin in some ancient upward thrust of the walls on the west side of the ancient fault.

On the eighteenth of April the trouble began in the sea. Just where, we may find out later. We know that the center is in the sea, because where the rift enters the land it was broader and its effects more violent than at any other point along its extent. As the rift can be traced for 192 miles across the land to the southward from Point Arena, it is safe to say that it goes as far to the northward under the sea. A steamer crossing it the moment of the earthquake, off Mendocino, ninety miles to the northward of Point Arena, bears witness to this fact. The captain thought that he had struck a raft of logs, so fierce and hard were the shocks of the waves in the water. . The movements were short, quick and violent, not forming a tidal wave, but a strange choppy sea. For the time being all rollers and surf were broken up. Off the bold headland of Cape Mendocino is a deep