THE NATURAL HISTORY SURVEY. 35 



are p (2131 ), 04041, and /oioTi. The angle between 2131 and 3121 

 is nearly lacking. The mean of three measurements gave for 

 the angle between 2131 and 2131 74; between loll and TIOI 

 75 i and between loll and 4041 35. Cleavage traces parallel 

 to oTi i appear on 3121 and edges of the rhombohedrom 0411 on 

 2131. 



Some of the best developed crystals are those which were 

 found on the east wall of the quarry at Stony Island. A view of 

 this quarry looking north is given in PI. VIII, fig. 2. These crys- 

 tals have been formed in the cavities in the limestone, and after 

 their deposition was complete they were covered and protected 

 by the bituminous residue of the petroleum which passed 

 through the same cavities. The hydrocarbon compounds oc- 

 cupied the cavities subsequently to the calcite mother liquid, 

 .since in no case has the asphalt been enclosed in a calcite 

 crystal. The scalenoheclrons stand out through the asphalt 

 which surrounds their base PI. IX, fig. I. They are rounded, 

 pitted and striated, showing that their growth has been irreg- 

 ular and that as deposition began upon one spot on a face, a 

 current was started in that direction, so that the molecules were 

 carried through the watery solution toward that point, causing 

 a more rapid growth of that plane. As the material upon that 

 radius became more attenuated, a new current would be estab- 

 lished from a direction of greater concentration of calcium 

 carbonate and a new plane would grow rapidly. Thus ridged 

 planes were produced. Subsequently, the planes were attacked 

 by carbonated waters poor in calcium, which produced etching 

 and corrosion of the faces and edges. 



Sometimes the cavities in the limestone are formed by the 

 dissolution of fossil shells that once occupied the rock. The 

 right hand specimen in PI. IX, fig. 2 shows a calcite crystal 

 partly filling a cavity once occupied by a zaphrentis. This 

 crystal illustrates the manner of crystal growth. The lower 

 portion was so rapidly formed as to be without definite shape. 

 As deposition progressed, it took place more slowly and the 

 upper part of the crystal was well formed. Usually crystals 

 crowd upon each other so that the bounding planes are obliter- 

 ated, as is shown in the left hand figure in PI. IX, fig. 2. The 

 cleavage of calcite parallel to R is perfect. Its fracture is 

 brittle. Its gravity 2.6 to 2.8. Its hardness 3. It may be 

 colorless or white or gray, blue green, yellow, red, brown, 

 black. The chief coloring materials are salts of iron, copper, 



