Ch. xxxviii.j chemical deposits in veins. 775 



slid to the right, we obtain considerable variation in the cavities so 

 produced, two long irregular open spaces, //, fig. 768, being then 

 formed. This wiU serve to show to what slight circumstances con- 

 siderable variations in the character of the openings between unevenly 

 fractured surfaces may be due, such surfaces being moved upon each 

 other, so as to have numerous points of contact. 



Most lodes are perpendicular to the horizon, or nearly so ; but some 

 of them have a considerable inclination or " hade," as it is termed, 

 the angles of dip varying from 15° to 45°. The course of a vein is 

 frequently very straight ; but if tortuous, it is found to be choked up 

 with clay, stones, and pebbles, at points where it departs most widely 

 from verticality. Hence at places, such as a, fig. 769, the miner com- 

 plains that the ores are " nipped," or greatly reduced 

 in quantity, the space for their free deposition having Fi £ 769 - 

 been interfered with in consequence of the preoccu- 

 pancy of the lode by earthy materials. When lodes 

 are many fathoms wide, they are usually filled for the 

 most part with earthy matter, and fragments of rock, 

 through which the ores are much disseminated. The 

 metallic substances frequently coat or encircle detached 

 pieces of rock, which our miners call "horses" or 

 "riders." That we should find some mineral veins 

 which split into branches is also natural, for we observe 

 the same in regard to open fissures. 



Chemical Deposits in Veins. — If we now turn from the mechanical 

 to the chemical agencies which have been instrumental in the produc- 

 tion of mineral veins, it may be remarked that those parts of fissures 

 which were not choked up with the ruins of fractured rocks must 

 always have been filled with water ; and almost every vein has prob- 

 ably been the channel by which hot springs, so common in countries 

 of volcanoes and earthquakes, have made their way to the surface. 

 For we know that the rents in which ores abound extend downwards 

 to vast depths, where the temperature of the interior of the earth is 

 more elevated. We also know that mineral veins are most metallifer- 

 ous near the contact of plutonic and stratified formations, especially 

 where the former send veins into the latter, a circumstance which 

 indicates an original proximity of veins at their inferior extremity to 

 igneous and heated rocks. It is moreover acknowledged that even 

 those mineral and thermal springs which, in the present state of the 

 globe, are far from volcanoes, are nevertheless observed to burst out 

 along great lines of upheaval and dislocation of rocks.* It is also 

 ascertained that all the substances with which hot springs are impreg- 

 nated agree with those discharged in a gaseous form from volcanoes. 

 Many of these bodies occur as veinstones ; such as silex, carbonate of 

 lime, sulphur, fluor-spar, sulphate of barytes, magnesia, oxide of iron, 



* See Dr. Daubeny's Volcanoes. 



