120 ANNALS NEW YORK ACADEMY OF SCIENCES 



from which crystals have been removed. Before a detailed study was 

 made, such effects were ascribed to ordinary processes of weathering, but 

 they occur repeatedly in material in which oxidation and other normal 

 effects of weathering are lacking, and it is frequently seen that later 

 minerals have been deposited in the cavities formed by the removal of 

 earlier. 



The ferromagnesian minerals present a series parallel to the feld- 

 spathoid compounds, but in these rocks their quantity is too small to form 

 the basis of an investigation similar to that which has been applied to the 

 others. They present a number of interesting features, however, which 

 will be described. 



The phase rule supplies no information as to sequence of deposition, 

 but simply shows how many phases can coexist in equilibrium. Never- 

 theless, if conditions were the same throughout the region, a sequence 

 which has been determined in an}' instance should apply to all cases. In 

 general, this appears to be true, but occasionally exceptions occur, imply- 

 ing local variations in concentration. This should be expected in a sys- 

 tem of circulation of the kind described, depending probably upon the 

 restrictions or freedom of flow and upon the tendency of minerals toward 

 segregation. 



Effects of this kind are exhibited especially by the ferromagnesian 

 compounds. It is found, for instance, that the deposition of pleochroic 

 green amphibole was conditioned largely by questions of concentration; 

 and that, depending probably upon circulation becoming more free, 

 amphibole which had been deposited was dissolved again at an early 

 period, while under other circumstances its deposition was still con- 

 tinuing. 



Another example is furnished by certain observations on analcite. 

 From the prevailing evidence, it appears that in general analcite was one 

 of the first of the zeolites to form, and, in several slides, it is found that 

 chabazite and heulandite encroach upon it. On the other hand, a speci- 

 men has been found in which crystals of analcite rest upon heulandite in 

 such a manner as to seem of necessity to imply later deposition. In 

 this case, some local enrichment of the solutions in soda or impoverish- 

 ment in lime is suggested. In later pages some inquiry will be made as 

 to how far the sequence of minerals which has been observed should be 

 considered an invariable one. 



The processes of alteration may be divided along the lines indicated 

 into two periods, which with their characteristic minerals are as follows : 



