176 ANNALS NEW YORE ACADEMY OF SCIENCES 



The possibility was also considered that while the majority of the 

 minerals originated through the form of circulation described, the forma- 

 tion of calcite should be assigned to a far later period, in fact to the 

 present cycle of events, in which cold surface waters carrying COj in 

 solution leach directly downward. The vital objection to this conception 

 is that such waters would also be oxidizing in nature, and the associations 

 of the greater part of the calcite render this idea very improbable. 



The hypothesis which was finally adopted as being most probable was 

 that the meteoric waters of the general form of circulation, which has 

 been accepted as responsible for the series of earlier minerals Avhich we 

 have considered, retained some store of the originally dissolved carbonic 

 acid (probably as bicarbonate of lime, CaHg (003)2, and carbonates of 

 the alkalies) at the period of their history at which they entered the 

 basalt sheet on the return journey to the surface. When conditions be- 

 came favorable, this supply of dissolved carbonic acid reacted with the 

 various original and secondary lime compounds and gave rise to the 

 formation of calcite. Carbonation, as is well known, is one of the most 

 common effects of the action of meteoric waters upon lime silicates at 

 moderate temperatures. When the temperature is high, on the contrary, 

 the process is reversed, so that carbonate rocks carrying silica, passing 

 from the zone of katamorphism into the zone of anamorphism, where 

 high temperatures prevail, have the carbonic acid driven off, while the 

 lime combines with silica and produces lime silicates. The process is, 

 therefore, evidently an easily reversible one under variations of tempera- 

 ture. This probably accounts for the observed fact that little or no cal- 

 cite appears to have been formed in the Watchung series during the 

 prevalence of the more elevated temperatures under which the minerals of 

 the first and second periods were deposited, and it was not until very 

 moderate temperatures were reached that carbonation became the chief 

 feature, tending to destroy the previously formed silicates. jSTaturally, no 

 sharp border-line can be drawn between the periods. For a time more oj* 

 less prolonged after the deposition of calcite had begun, the formation of 

 silicates continued. Laumontite and apophyllite were deposited simul- 

 taneously with calcite almost beyond question. It is not impossible that 

 some slight deposition of calcite began at an earlier stage, but decisive 

 evidence of this is lacking. The fact must not be overlooked that the 

 temperature at which deposition of calcite began would be a function of 

 the concentration of the carbonic acid in the solutions. Under the con- 

 ditions which seem to have existed in our rocks, the quantity of carbonic 

 acid was probably minute, and the large amount of calcite found must 

 be referred to the continued passage of the water for a prolonged period. 



