FENNER. THE WATCHUNG BASALT 177 



Under different conditions, a much larger quantity of CO2 might be 

 present, and the formation of calcite would begin at an earlier date. For 

 example, in the not unusual zeolitic deposits in which the minerals form 

 distinct veins cutting sheets of basic eruptives, it is extremely probable 

 that circulation is generally begun while emanations are still being given 

 forth by the cooling magma and that these are added to the ascending 

 waters. Under such circumstances, the concentration of CO2 might be 

 supposed to reach a much higher value, and as a consequence, the period 

 of calcite deposition would be much advanced. In the zeolitic veins cut- 

 ting the great intrusive sill of Palisade diabase, it is observed that large 

 crystals of calcite are encrusted with analcite and stilbite. An earlier 

 period for calcite is indicated, which may thus be accounted for. In 

 specimens from other localities, also, with whose geological relations the 

 writer is not familiar, an early period of deposition of calcite has been 

 observed. 



Thaumasite and Gypsum 



At about the same period as calcite, a small amount of gypsum was 

 deposited and also the unusual mineral thaumasite. The latter is appar- 

 ently a combination of silicate, carbonate and sulphate of lime. Its 

 occurrence has been noted in only a few localities throughout the world. 

 Gypsum has been observed resting apparently upon calcite crystals, and 

 masses of thaumasite are found to contain nodules of pectolite, heulan- 

 dite and apophyllite, imbedded like raisins in a pudding. These rela- 

 tions indicate a late period of deposition for the two minerals. The sul- 

 phates necessary for their formation can likewise be considered referable 

 to meteoric sources. 



During the calcite period, green amphibole, chlorite, specularite and 

 probably pyrite and chalcopyrite were also deposited as has been described. 



The presence of gypsum affords an approximate mark on the thermo- 

 metrical scale. Van't Hoff and his associates^* have made elaborate in- 

 vestigations on the relations of gypsum and anhydrite. They fixed the 

 transition point between gypsum and natural anhydrite at 63.5° C. and 

 175 mm. vapor pressure. The presence of other salts in solution some- 

 what affects the value of the transition point. In a saturated solution of 

 NaCl, they decided upon a mean value of 30° for the temperature of 

 formation of anhydrite from gypsum. In our solutions, not all the con- 

 ditions are known, but it seems certain that if the temperature at this 

 time had exceeded 63.5° C, the sulphate of lime would have been de- 

 posited in the form of anhydrite instead of gypsum. 



"Zeltschr. phys. Chem., vol. 45, pp. 257-306, 1903. 



