Barrett — Physical Effects of Contact Metamoi'phism. 295 



marbles are isolated from the adjacent sedimentaries by erup- 

 tions of andesite.* These blocks are 2000 to 3000 feet long 

 and 50 to 150 feet thick. The lower, which is separated irom 

 the upper by an andesite sheet 60 feet in thickness, is tilted at 

 an angle of 18° to the horizontal. Within this lower marble 

 is a three-inch band of light-colored hornstone, whose mineral 

 composition was estimated by the microscope as follows, indi- 

 cating a loss of 35 to 40 per cent in volume. 



Per cent. 



Diopside 50 



Anorthite and labradorite _ _ 46 



Quartz ._ _ 2 



Fluorite . _ 2 



100 



The quartz and fluorite are scattered over the section in 

 branching nuclei and surrounding them the diopside assumes a 

 considerable coarseness of crystallization. The process which 

 has evidently gone forward here is, flrst, a crystallization of 

 diopside and feldspar which has left the stratum very porous. 

 Then quartz was deposited around the cavities, which were 

 finally filled by fluorite brought in by gases escaping from the 

 magma. 



The second instance is the ore stratum of the Dolcoath 

 mine,f situated a quarter of a mile northwest of the town of 

 Elkhorn, Montana. 



This is a bed of altered limestone 15 to 18 inches in thick- 

 ness, dipping 55° east and carrying gold together with bismuth 

 sulphide and telluride. An examination of thin sections of 

 the ore-bearing stratum and also the foot wall and hanging 

 wall, gave the following results, the figures on account of the 

 nature of the sections being only approximate. 



Ore Stratum. Foot wall. Hanging wall. 



Diopside 45 Diopside 30 Augite 5 



Garnet 40 Garnet 10 Biotite 25 



Calcite 12 Basic feldspar. 60 Basic feldspar, 66 



Sulphides, with Sulphides, no 



gold __„.-__ 3 gold 4 



100 100 100 



It is seen that even if all the calcite of the ore-bearing 

 stratum be regarded as a primary mineral, the shrinkage in 



* Elkhorn Mining District, Montana, by Walter Harvey Weed, TJ. S. Geol. 

 Survey, 21st Ann. Keport, p. 449. 

 fLoc. cit., p. 506. 



