Page:The fairy tales of science.djvu/315

 the principal metal, frequently contains small quantities of silver, zinc, and cobalt; manganese is often associated with iron, while platinum is usually mixed with gold. Besides the ores of metals, these veins almost always contain quartz, fluorspar, crystalline carbonate of lime, and other spars.

"Ores and spars, however, are not confined to the deep fissures that occur in the earth's crust. They find their way into all kinds of cracks and cavities, whatever may have been the cause of the hollows, and even into detached holes, often no larger than your fist, and completely surrounded by solid rock. Wherever, indeed, permanent hollows and interstices of any kind, size, shape, or origin exist in hard rocks, crystallized minerals, spars, and ores may be formed in them.

"How do these matters reach the cavities, is a problem which you will perhaps expect me to solve, but if so you will be disappointed. A number of clever mortals are striving to arrive at the true solution of this mysterious question, and were I to tell you all I know, I should be robbing some future philosopher of the fame that will accrue from a great discovery. I will, however, give you one or two hints, which may help you to form some conception of the mode in which the veins and isolated cavities may be filled.

"Look around at these walls of crystal, these pillars of porphyry, this floor of marble, and these hanging stalactites! All these things have been