358 M. Amedee Burat on the 



below, which were but little interrupted except by contrac- 

 tions. 



Such is the structure, as has been mentioned, of the veins 

 of Clausthal in the Hartz : such is likewise that of the great 

 argentiferous veins of Mexico. It is a, practical rule much 

 followed in Cornwall, to seek for minerals always below rich 

 parts already known. In Algeria, considerable works have 

 been undertaken on veins of grey copper, veins altogether 

 vii'gin, in which this law has been first rendered evident. 

 "Works opened for some years in the veins near Villefranche, 

 have shewn that the minerals there constituted rather nar- 

 row zones, but that they were continuous according to the 

 depth. In some veins of Nassau, these zones were oblique, 

 that is to say, followed a diagonal between the direction and 

 the inclination. 



Do all these facts prove that works carried on to a great 

 depth are to be regarded as infallible ^ Certainly not ; these 

 works remain subject to the accidents of mines, in which no- 

 thing is absolutely certain. They demonstrate, however, that 

 the great principles of continuity in depth ai'e founded on ex- 

 perience. They also demonstrate that, in the case of an un- 

 successful attempt, we must not pronounce absolutely, but 

 examine whether the vein we suppose to be terminated in 

 depth, has not merely undergone one of those accidents which 

 the theoretical and practical study of the science make known 

 to us. 



The veins of Holzappel were for a long time considered as 

 closed in depth, at a certain part of their course. A more 

 attentive study shewed that these veins had merely under- 

 gone a lateral displacement of from 10 to 15 metres, thus 

 passing from one cleavage to another, and that the two pai'ts 

 were united in such a manner as to escape notice on the 

 first examination. This example, so well described by M. 

 Bauer, has removed the only solid objection made to con- 

 tinuity, and the existence of minerals again met with below 

 these changes of allure, strengthens, by another additional 

 fact, great geological principles. 



Let us now examine the conditions of allure in some irregu- 

 lar repositories, and we shall witness the principle of the 



