158 ANNALS NEW YORK ACADEMY OF SCIENCES 



turbidity of the pectolite in the vicinity and the manner in which the 

 fibres disappear in the chabazite. At one point, the pectolite area is cut 

 across by small veins of chabazite and calcite. 



Fig. 23. Masses of pectolite needles and isolated remuauts of same in the midst 



of apophyllite. X 35. Slide 129. 



In the hand specimen from which 86 was cut, crystals of chabazite 

 rest upon the hemispherical surfaces of pectolite masses. In fig. 23, the 

 replacement of pectolite by apophyllite in 129 is sketched. Slide 135 

 shows practically the same relations. 



General Results of the First Period of Alteration 



In the slides which have been described, in which the minerals belong- 

 ing to the first period of alteration are best developed, there are com- 

 monly but obscure indications as to the nature of the original material 

 which has been replaced. Mention has been made, however, of certain 

 circular figures and curved lines and markings, due apparently to an 

 insoluble residue of TiOj, etc., upon which the forces of recrystallization 

 had no effect. These have been interpreted as survivals of an original 

 structure in the glass. Examples are shown in Plate XI, fig. 5, Plate 

 XII, fig. 5, and Plate XIII, fig. 2. There are instances, too, in which 

 the secondary minerals are separated from the basalt of normal texture 

 by a marginal band in which the texture is of much finer grain. This is 

 believed to represent a transition between glassy crusts and more slowly 

 cooled interiors. In all these cases the original glass has totally disap- 

 peared, and with it such phenocrysts of diopside and plagioclase as may 

 have been present, but in a few instances it is seen that portions of it 

 survived. 



