Suggest how rocks now in process of metamorphism may sometimes be exposed to view. Why do metamorphic rocks appear on the surface to-day? Mineral Veins In regions of folded and broken rocks fissures are frequently found to be filled with sheets of crystalline minerals deposited from solution by underground water, and fissures thus filled are known as _mineral veins_. Much of the importance of mineral veins is due to the fact that they are often metalliferous, carrying valuable native metals and metallic ores disseminated in fine particles, in strings, and sometimes in large masses in the midst of the valueless nonmetallic minerals which make up what is known as the _vein stone_. The most common vein stones are _quartz_ and _calcite_. _fluorite_ (calcium fluoride), a mineral harder than calcite and crystallizing in cubes of various colors, and _barite_ (barium sulphate), a heavy white mineral, are abundant in many veins. [Illustration: Fig. 258. Placer Deposits in California _g_, gold-bearing gravels in present river beds; _g´_, ancient gold-bearing river gravels; _a_, _a_, lava flows capping table mountains; _s_, slate. Draw a diagram showing by dotted lines conditions before the lava flows occurred. What changes have since taken place?] The gold-bearing quartz veins of California traverse the metamorphic slates of the Sierra Nevada Mountains. Below the zone of solution (p. 45) these veins consist of a vein stone of quartz mingled with pyrite (p. 13), the latter containing threads and grains of native gold. But to the depth of about fifty feet from the surface the pyrite of the vein has been dissolved, leaving a rusty, cellular quartz with grains of the insoluble gold scattered through it. The _placer deposits_ of California and other regions are gold-bearing deposits of gravel and sand in river beds. The heavy gold is apt to be found mostly near or upon the solid rock, and its grains, like those of the sand, are always rounded. How the gold came in the placers we may leave the pupil to suggest. Copper is found in a number of ores, and also in the native metal. Below the zone of surface changes the ore of a copper vein is often a double sulphide of iron and copper called _chalcopyrite_, a mineral softer than pyrite--it can easily be scratched with a knife--and deeper yellow in color. For several score of feet below the ground the vein may consist of rusty quartz from which the metallic ores have been dissolved; but at the base of the zone of solution we may find exceedingly rich deposits of copper ores,--copper sulphides, red and black copper oxides, and green and blue copper carbonates, which have clearly been brought down in solution from the leached upper portion of the vein.