1 84 
The American- Geolos[ist. 
September, 1902 
Tt would be tedious to enumerate all the familiar deposits 
in limestone. Those of Leadville, Colorado, and Eureka, Ne- 
vada, have been thoroughly studied and described. Tombstone, 
Arizona, presents ([uartzose veins tilliui^ fault-fissures, cutting 
slightly tilted sedimentary rocks, with the workable ore-bodies 
formed b)' replacement of limestone along bedding-planes, and 
presumably by the same solutions that filled the fissures. 
Derbyshire. — The Derbyshire lead-mines of England are 
also well-known examples of veins carrying galena in lime- 
stone, and barren when in the intercalated intrusive trap-rocks 
(toadstone.) Fig. 8, from De la Beche, illustrates the occur- 
rence of galena in the limestones above and below an intru- 
sive sheet in one of the Derbyshire mines. The leadu" or fissure 
traverses the "toadstone" as well as the lime-rock, but it is only 
in the limestone that galena occurs in the altered area. The 
channels or rakes {g i, d k, c /;;,) were fissures, through which 
solutions reach the pi])es or ore-bunches p p, the bedding-plane 
deposits f f, and the joint deposits li h. 
Fig. b 
Limestone Beds of Derbyshire, with Intercalated Beds of Igneous 
]\ock (Toadstone) Traversed by \'eins. (De la Beche.) 
Contact-Deposits. 
As Lindgron has remarked in discussing the formation of 
contact-deposits, a chemical reactioti seems to take place be- 
tween the substances leaving the magma and the carbonate of 
lime, causing the deposition of new minerals and the liberation 
of carbon dioxide. In the cases mentioned in this paper, the 
evidence shows that the veins were probably formed, not by 
true pneumatolitic action, but by hot circulating waters which 
were as trulv a result of the igneous intrusion as the vapors 
and gases of pneumatolitic action. The escaping vapors and 
gases from the magma were, it is believed, taken itp l\v circu- 
lating waters of either deep-seated or meteoric origin, so that 
the "mineralizing agents" which had taken up and formed the 
