ART. 28 MINERALOGY OF TRIASSIC LIMESTONE SHANNON 7 



of the original conglomeratic structure, remnants of the original 

 pebbles showing either as deeper green or browner green masses or 

 as incompletely replaced granular areas richer in calcite than the 

 matrix. Under the microscope this is seen to consist predominantly 

 of diopside, with less calcite, garnet, and serpentine. The diopside 

 is fine granular, colorless, and of normal optical properties, and is 

 anhedral except where it projects into calcite or serpentine there 

 forming short, stout prisms. The garnets are hexagonal in outline 

 and are anomalously birefracting, some with division into sectors. 

 The centers of some are isotropic, the outer border having a rela- 

 tively high birefringence, while others show a uniform low order 

 blue interference color. Occasionally they have yellow-brown cores. 

 The serpentine forms fine flaky colorless interstitial areas or fills 

 cracks in the diopside, and is probably the latest mineral in the 

 section. 



" Lb-4 " is a massive granular lusterless rock like the last but of 

 a more yellowish green tone. Pebbles of the original structure are 

 shown by masses of more yellowish color dotted with dark specks. 

 This contains much less visible calcite than " Lb-3." Under the 

 microscope this rock is also found to consist principally of granular 

 diopside, with large poikilitic crystals of a uniaxial positive mineral 

 of low birefringence and high refractive index which is probably 

 vesuvianite. Groups of small colorless isotropic garnets and a little 

 flaky interstitial serpentine occur. 



"Lb-5 " is a dense fine-grained rock having a greenish-gray to 

 lilac-gray color dotted with dark spots only about 0.2 mm. in size, 

 which give the rock a speckled " pepper-and-salt " appearance. 

 Under a lens these dark spots, which look like minute manganese 

 oxide stains, are seen to be resinous and lustrous. They are small 

 patches of garnet. Under the microscope this rock is found to be 

 composed of calcite, diopside, and garnet in roughly equal amounts. 

 The calcite, which is a fabric of coarse interlocking grains, forms a 

 matrix in which the other minerals have developed, probably by 

 replacement. The diopside is colorless and of normal optical proper- 

 ties. It occurs as large ragged and irregular crystals inclosing much 

 calcite and also as radial aggregates of slender prisms. The garnet 

 forms granular areas, ragged in outline and including much diopside. 

 It is completely isotropic, and varies from colorless to resinous 

 brown. 



One section was cut showing the actual contact between glassy 

 chilled basalt and the lime-silicate rock. The latter is made up of 

 coarse and fine granular diopside, sharply euhedral garnets, scattered 

 large vesuvianite grains and interstitial patches of serpentine and 

 calcite. The body of the rock and the earliest mineral now shown 



