There are two great types of place where gold is found. In the alluvial deposits, the beds of old rivers, the gold is quite loose. The convulsions of ages ago have, in many cases, elevated these beds, until now they are on the sides of mountains. In such cases the loose, gravelly stuff of which they are composed is washed down by a powerful stream of water from a huge hose-pipe terminating in a nozzle called a "monitor." This process, called "hydraulicing," brings down everything into a pond formed at the foot of the hill, and in some cases a boat or raft is floated upon the pond with machinery on board for dredging up the material. Often a powerful centrifugal pump sucks up the water through a pipe reaching to the bottom of the pond, bringing gravel and gold with it. Arrived in this way upon the raft, it all goes on to separating tables, by which the gold, being heavier, is divided from the gravel, which is lighter. These tables will be referred to again later. In non-alluvial workings the gold is embedded in rock of some kind, such as that called quartz. This is hard, somewhat of the nature of granite, and before the gold can be liberated it has to be crushed to the likeness of fine sand, so that the tiny grains of gold can be captured. The quartz is found in veins or lodes, fissures, evidently, in the original crust of the earth, produced probably as the earth cooled. These have been gradually filled up by hot volcanic streams of water, which carried not only the gold in solution but also the materials of which the quartz is formed. It used to be thought that the veins were the result of hot liquids forced up from below by volcanic action, the rock and metal being themselves in the liquid state through intense heat. It is now more generally held that water was the vehicle by which the materials were brought in, and the vein formed. The gold in the alluvial deposits, too, is now thought to have come there in solution in water, and not by the erosion and washing down of rocks higher up the original river.