Deep leads are detected at deep places below the surface, often covered by beds of lava, which may be many in number and hundreds of feet in thickness. The deep lead is an ancient river bed gold is derived from metamorphic crystalline rocks of the range, partly from quartz veins in the slates and partly from gold distributed in minute quantities all through the metamorphic rocks. The quartz veins lie between the planes of stratification of the slates, also in irregular bunches and lenticular masses of limited extent. In some places, the rocks are penetrated in every direction by little irregular quartz veinlets, which often contain gold and in spots extremely rich, even where quartz vein is not very thick. In some California places where a basalt capping exists, the drift beneath contains gold.
The modes of occurrence of auriferous gravel deposits are variable. Sometimes, they exist in well defined ancient river beds under a capping of basalt which has filled the channels of the rivers in the past time. Again, they appear in isolated mounds or hillocks, evidently they remain of such channels, which being unprotected by a covering of lava, have been broken up by the action of the elements, also in basins or flats that have received the wash of these disintegrating rivers. Also, they are detected in low rolling hills near the base and beyond the reach of the lava flows.
In some deep leads deposits, the waters percolating through the lava flows and reaching the gravels beneath are charged with alkali from the lava. These alkaline waters contain silica in solution from the same source. Hence, the fossil drift of these ancient rivers has all been silicified by these siliceous waters. The gravels are also cemented by the same material. The waters also contain iron. In this iron cement, gold is found in rounded grains and in minute crystals and threads deposited by a solution of iron sulphate at the moment of the reduction of the latter to a sulphide. Gold prospectors have noted that streams that were richly gold –bearing up to certain point, increasing as this point was neared but ceasing when it was passed. These parts are in line of different streams and following up a course, the lead was eventually struck on several sections and tunneled on.
Some gold deposits at 300 feet deep are composed of gravel, boulders, clay and sand, on strata distinguished by degrees of fineness by the character of the rocks and the gold content, also by colors such blue-gray. Basically, gold may be coarser near the bottom and presence of silver is common. The silver in the gold in the upper stratum has been eaten out by sulphurous acid resulting from decomposition of pyrite. The whole deposit is like that in existing rivers, showing banks, eddies, falls, rapids and riffles. Typically, there is much gold in eddies, but little in rapids. Where dead rivers meet, the wash is usually rich. Where lead becomes very narrow, dips fast and is trapped between steep walls, the gold will be very sparingly distributed in holes and behind ridges and will be coarse.