ART. :.'8 MINERALOGY OF TEIASSIC LIMESTONE SHANNON 31 



the basalt dikes as described above. It forms an irregular mass 

 15 millimeters in maximum diameter, varying from chalky white 

 at the borders to translucent purplish gray in the center. It has 

 the usual lusterless surface and flinty fracture. 



EXPLANATIONS OF PLATES 



Plate 1 



Replacement of limestone conglomerate along a narrow fissure. Shows the 

 replacement of the calcareous sand forming the matrix of the pebbles by a 

 mixture of diopside vesuvianite and garnet and, nearer the fissure, later re- 

 placement by magnetite. Two-thirds natural size. 



PlATE 2 



Crust of datolite crystals encrusting the walls of a narrow open fissure in 

 limestone. Natural size. 



Plate 3 



Upper left : Rectangular mold of anhydrite crystal preserved in datolite. 

 Niifural size. 



Upper right: Thin platy molds of anhydrite crystals preserved in calcite. 

 Natural size. 



Lower: Platy barite resting on datolite crystals. Natural size. 



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