Petroleum from the fountains of Is, on the banks of the Euphrates about 120 miles from Babylon, furnished the asphaltic mortar for building Nineveh 2,000 years before the Christian era. There are many sources of naphtha, petroleum, and asphalt in Europe and Asia, which like those in Trinidad and Venezuela occur for the most part in rocks of the newer, secondary and tertiary formations, though sometimes in the lower. But in the northern part of the United States and Canada these substances occur in rocks of all ages from the lower silurian to the tertiary period inclusive; they are usually found in the limestones and more rarely in the sandstones and shales. Petroleum collects in the fissures of the rocks, chiefly in those that have a tendency downwards; in wells dug for it near one another, an abundant supply is furnished at all depths from 70 to 300 feet. In some parts of Ohio and Canada the ground is saturated with petroleum, so that it is believed there is enough in North America to supply the world for ages. In 1861 no less than 42,000,000 gallons of petroleum were sent to England. The wells are not without danger, for when they pass through the coal strata, the petroleum is accompanied by a highly inflammable gas which on one occasion was accidentally set on fire; it ignited the petroleum, which was forced out as from the mouth of a volcano, and covered the ground with liquid fire far around; at the same time the burning gas formed an incandescent atmosphere which extended to a still greater distance.