8 PROCEEDINGS OF THE NATIONAL MUSEUM vol. G6 



by the section is diopside, which includes the later large ragged and 

 poikilitic vesuvianites. The garnets have sharp outlines where they 

 abut against calcite or serpentine and are isotropic except at the 

 borders, where they have a narrow double refracting outer layer. 

 They are grouped in a manner indicating that they probably de- 

 veloped lining minute cavities which were later filled with serpen- 

 tine now largely replaced or saturated with still later calcite. 



The replacement of the limestone by the high temperature solu- 

 tions moving along fissures is Avell shown by the specimen illus- 

 trated in plate 1. The solutions were controlled by narrow frac- 

 tures, seldom of any significant size. Along some of these there is 

 some crushing and slickensiding indicating some movement, but in 

 others they are simply weak cracks which have not been accom- 

 panied by any displacement at all. They vary somewhat in attitude 

 and dip, ranging from some 60° to vertical, and in general have a 

 north-south strike. Adjacent to this crack the limestone has been 

 replaced by lime silicates, principally diopside with less garnet and 

 serpentine, and some vesuvianite. This replacement extends to vari- 

 able distances from the fissure. In the illustrated specimen the width 

 of the central filled crack averages only about 2 millimeters, yet the 

 replacement with development of abundant diopside reaches a dis- 

 tance of 10 centimeters from the crack and abundant magnetite has 

 developed up to 4 centimeters away. The specimen is composed pre- 

 dominantly of two kinds of limestones in the usual sandy cement. 

 The matrix of the pebbles has been preferentially replaced by the 

 lime silicates while at the same distance from the fracture the coarse- 

 grained gray-and-white mottled marble has not been attacked at 

 all while a finer granular buff-white marble has been slightly re- 

 placed in porous streaks and along rifts. The controlling factor 

 in the replacement has apparently been permeability. Near the 

 fissure many of the fragments of limestone which did not yield to 

 the alteration to lime silicates have been impregnated with fine scales 

 of serpentine in concentric layers parallel to their outer surface. 

 The magnetite has not replaced the lime silicates to any great extent 

 but has developed principally by replacement of these serpentinized 

 limestone masses, the structure of the replaced marble being retained 

 in the structure of the magnetite. 



The lime silicate rock is not so well individualized in these small 

 replacements as in the large diopside rock masses previously de- 

 scribed, the garnet being in the form of irregular and indistinct 

 patches. 



The central crack is lined with a layer of about 1 millimeter of 

 diopside, overlain by a layer of minute magnetite grains following 

 which the remaining open space was filled with coarse white calcite. 



