Part 1 — THE LAVA FLOOD OF THE NORTHWEST

We know little about the substance that occupies the four thousand miles of distance between the surface and the centre of our earth. We know that the terrible weight borne by the central mass compresses it, so that the interior must grow denser as the core is approached. Scientists have weighed the earth, and tell us that the crust is lighter than the rest. The supposition is that there is a great deal of iron in the interior, and possibly precious metals, too.

Our deepest wells and mines go down about a mile, then digging stops, on account of the excessive heat. But the crumpling of the crust, and the wearing away of the folded strata by wind and running water, have laid bare rocks several miles in thickness on the slopes of mountains, and exposed the underlying granite, on which the first sedimentary rocks were deposited. On this granite lie stratified rocks, which are crystalline in texture. These are the beds, sometimes miles in depth, called metamorphic rocks, formed by water, then transformed by heat.

The wearing away of rocks by wind and water has furnished the materials out of which the aqueous rocks have been made. Layers upon layers of sandstone, shales, limestone, and the like, are exposed when a river cuts a canyon through a plateau. The layered deposits of débris at the mouth of the river make new aqueous rocks out of old. Every sandy beach is sandstone in the making. This work is never ended.

In the early days the earth's crust often gaped open in a mighty crater and let a flood of lava overspread the surface. The ocean floor often received this flood of melted rock. In many places the same chimney opened again and again, each time spreading a new layer of lava on top of the old, so that the surface has several lava sheets overlying the aqueous strata.

If the hardened lava sheet proved a barrier to the rising tide of molten lava in the chimney it was often forced out in sheets between the layers of aqueous rocks. Wherever the heated material came into contact with aqueous rocks it transformed them, for a foot or more, into crystalline, metamorphic rocks.

A chimney of lava is called a dike. In mountainous countries dikes are common. Sometimes small, they may also be hundreds of feet across, often standing high above the softer strata, which rains have worn away. Dikes often look like ruined walls, and may be traced for miles where they have been overturned in the mountain-making process.

The great lava flood of the Northwest happened when the Coast Range was born. Along the border of the Pacific Ocean vast sedimentary deposits had accumulated during the Cretaceous and Tertiary Periods. Then the mighty upheaval came, the mountain ridge rose at the end of the Miocene epoch and stretched itself for hundreds of miles through the region which is now the coast of California and Oregon. Great fissures opened in the folded crust, and floods of lava overspread an area of 150,000 square miles. A dozen dead craters show to-day where those immense volcanic chimneys were. The depth of the lava-beds is well shown where the Columbia River has worn its channel through. Walls of lava three thousand feet in thickness rise on each side of the river. They are made of columns of basalt, fitted together, like cells of a honeycomb, and jointed, forming stone blocks laid one upon another. The lava shrinks on cooling and forms prisms. In Ireland, the Giants' Causeway is a famous example of basaltic formation. In Oregon, the walls of the Des Chutes River show thirty lava layers, each made of vertical basalt columns. The palisades of the Hudson, Mt. Tom, and Mt. Holyoke are examples on the eastern side of the continent of basaltic rocks made by lava floods.

Northern California, northwestern Nevada, and large part of Idaho, Montana, Oregon, and Washington are included in the basin filled with lava at the time of the great overflow, which extended far into British Columbia. It is probable that certain chimneys continued to discharge until comparatively recent times. Mt. Rainier, Mt. Shasta, and Mt. Hood are among dead volcanoes.

Quite a different history has the great Deccan lava-field of India, which covers a larger area than the basin of our Northwest, and is in places more than a mile in depth. It has no volcanoes, nor signs of any ever having existed. The floods alone overspread the region, which shows no puny "follow-up system" of scattered craters, intermittently in eruption.