ART. 28 MINERALOGY OF TEIASSIC LIMESTONE SHANNON 9 



Thin sections from a second similar vein show the same relations. 

 The central crack is filled with granular calcite containing dissemi- 

 nated magnetite grains and bordered by several alternate layers of 

 diopside and magnetite. There are also layers of another bladed 

 fibrous mineral of low birefringence, with an index of refraction 

 of about 1.56. This mineral is optically positive and probably 

 uniaxial. In optical properties it agrees with brucite or colerainite. 

 It is probably a white chlorite allied to colerainite. Small cavities 

 in the rock adjacent to the crack are lined with a botryoidal brown 

 layer and filled centrally with pale yellow to colorless material 

 which is isotropic at the borders to feebly birefringent with a fine 

 confused fibrous structure at the center. These have the appearance 

 of opal and chalcedony. 



Another specimen shows abundant magnetite associated with the 

 colorless chloritic material, and large anhedral areas of garnet 

 which is colorless and isotropic and grades into a thick layer of 

 garnet coating a slickenside along the parent crack. This garnet 

 is largely replaced by a golden brown isotropic material of high 

 refractive index which tends, in places, to form spherical globules 

 each of which has a minute nucleus which appears to be a colorless 

 octahedral crystal. 



The minerals which occur as constituents of what are here called 

 high temperature hydrothermal replacements may now be enumer- 

 ated, with descriptions. 



DIOPSIDE 



Diopside is the most abundant of the minerals replacing limestone 

 and makes up large masses of secondary lime-silicate rock as de- 

 scribed above. It is always microscopic granular and never recog- 

 nizable with the unaided eye. In thin section it is colorless with 

 normal optical properties. Some of the masses of rock consisting 

 predominantly of diopside are a meter or two in diameter. 



VESUVIANITE 



Vesuvianite occurs only as scattered microscopic grains, conspicu- 

 ous in thin section but invisible to the unaided eye. It is a minor 

 constituent of the lime-silicate rocks. 



MAGNETITE 



Magnetite occurs as fine granular masses adjacent to fissures in the 

 limestone where it accompanies the various secondary silicates. It 

 has chiefly formed by replacement of limestone masses adjacent 

 to the fractures and is younger in age than the lime silicates and 

 serpentine. 



9098—25 2 



