GAGE IRON ORES IN MISSOURI. 191 



the magnetite deposits and specular iron-ore of this mountain. It 

 seems to be j'ounger than the dark liver-colored porphyry which 

 constitutes the mass of the mountain. I judge it to be younger 

 from the fact of finding fragments of the purple porphyry en- 

 closed in its mass near their line of contact. This red porphyry 

 has at some points a jointed structure, and along the seams I found 

 a purple mineral which I first thought Amethyst, but afterwards 

 judged to be Fluorspar from its degree of hardness, but have not 

 yet had the time positively to determine. 



The ores of Shepherd Mountain are highly magnetic, affecting 

 the needle (compass) far out into the valleys ; a large mass will 

 support heavy weights of iron. The ores of Pilot Knob and Cedar 

 Mountain, though not magnetic, possess polarity, and on the 

 south spur of Pilot Knob my needle was so strongly affected that 

 the solar compass had to be substituted. On the north-west slope 

 of Cedar Hill the compass was similarly affected. 



Now upon correlating all my facts, the result of several months' 

 investigation, I find (certainly much of this may be called theory, 

 for it does not admit of direct proof) that this region is composed 

 of mountains of porphyry, sometimes isolated as Shepherd Moun- 

 tain, sometimes several connected by saddles as Pilot Knob with 

 Buck and Peck Mountains, the intervening valleys covered with 

 limestone, chert, and sandstone, belonging mostly to the Lower 

 Silurian, a portion of the limestone (south of the Knob) belong- 

 ing to the Azoic age, the whole formation being devoid of any 

 fossil remains : the porphyries composing the mountains varying 

 in texture, sometimes with compact matrix, with and again with- 

 out crystals, possessing here a slaty texture, then bedded or stra- 

 tified, and again passing from a granular matrix into a conglom- 

 erate containing impregnations, segregations, veins and beds of 

 iron-ores, which ores I have every reason to believe have been 

 deposited — possibly (?) with the exception of the deposit on Shep- 

 herd Mountain — through replacement of the porphyry by lateral- 

 secretion when the porphyry masses were at great depth, and 

 later a subterranean force caused them to assume their present 

 position : by elevating some portions and depressing others a 

 rugged outline was given to the present shape of the mountains, 

 and since their elevation no violent force of nature has acted on 

 them, though rains and storms have taken an active part in giv- 

 ing the present contour lines by attacking material which offered 



