FENNER, THE WATCHVNG BASALT 175 



THIRD PERIOD OF ALTERATION 



Calcite 



Calcite occurs in quantities almost equal to all the other minerals 

 combined. In habit, it is very variable. The most common form of 

 crystal is probably the unit rhombohedron, but slender scalenohedrons 

 (dog-tooth spar) are frequently found, and complex combinations of 

 crystal faces are not unusual. It is also frequently developed in 

 finely granular masses. Its presence is so nearly universal that it is 

 rather rare to find a specimen of any mineral or group of minerals which 

 does not show the presence of calcite, if tested with acid. In the earlier 

 part of the work, it was found a hindrance, because of the frequency with 

 which it obscured the study of desired relations. It was finally found 

 necessary to apply the acid test in every case before making sections, in 

 order to make sure that the later introduction of calcite had not destroyed 

 the desired evidence. Its frequent presence necessitated the rejection of 

 a great quantity of otherwise available material, and in some cases re- 

 quired that a long search be made before specimens free from calcite 

 could be found. 



Reference has already been made to the manner in which granular 

 calcite of later growth frequently develops between crusts of earlier min- 

 erals and the less altered basalt and simulates earlier deposition. At 

 times, this takes the form of well-developed crystals, upon which the 

 older crusts appear to rest, but in such cases, microscopic examination 

 frequently shows certain abnormalities in the relations. 



In the great majority of cases, there can be little doubt that calcite is 

 of later formation than all the minerals of the feldspathoid series, with 

 the exception of laumontite. Occasionally its deposition may have begun 

 somewhat earlier, but this was a rather abnormal condition, and in 

 general, calcite is the replacing mineral. 



It was found difficult at first to account in a satisfactory manner for 

 the introduction of such quantities of carbonic acid as the abundance of 

 calcite requires. The writer was disposed to look for its sources in 

 vapors evolved from the magma, but although such vapors were probably 

 given forth, they would, like aqueous vapors, have opportunity under the 

 conditions of consolidation to dissipate freely in the air, unless they were 

 locked up in some nonvolatile combination until the late period at which 

 calcite appeared. Search was made for evidence of some intermediate 

 mineral to whose decomposition the carbonic acid of the calcite could be 

 ascribed, or for indications of an early appearance of calcite, but evidence 

 in both directions appeared to be entirely lacking. 



