AMYGDALOIDS. 89 
respect we have yet another affinity between the amygdaloids and the non- 
vesicular diabase-porphyrites. 
Macroscopically, the vesicles are seen to be commonly filled with 
secondary minerals—one or more of “calcite, chlorite, epidote, quartz, 
prehnite, laumontite, copper, orthoclase, or their products of alteration.”* 
Often, however, I have observed the vesicles empty, either from the removal 
of the amygdules or from their having always remained empty. The walls 
of these empty cavities are commonly found to be smooth and dense, 
apparently from the pressure of the confined vapor. 
Although a large number of sections of amygdaloids were cut with 
this object in view, I have not been able to find the time to extend the 
studies, so ably begun by Professor Pumpelly, of the changes which have 
brought about the fillings of the vesicles and the various stages of altera- 
tion of the matrix. He sums up the results of his studies on the alterations 
of both pseud-amygdaloids and the true vesicular amygdaloids in the fol- 
lowing table, which is designed to show the course and final results of the 
most common process of alteration: 
I. Hydration of chrysolite, when present. 
Il. Change of augite, loss of lime, and partial loss of 
; iron and magnesia. 
Pseud-amygdaloid stage. < III. Change of feldspar to prehnite, and formation of 
prehnite pseud-amygdules. 
IV. Change of prehnite to chlorite. 
IVa. Change of prehnite to orthoclase. 
( I. Filling gas vesicles with prehnite, or other minerals. 
Change of matrix to ferruginous prehnite. 
II. Change of the prehnite, in places, to chlorite; in 
Amygdaloid stage..---.- others, to calcite and green-earth; in others, to 
epidote and calcite. 
Ill. Entrance of quartz, filling all the interstices, and re. 
placing the calcite. 
The following are Pumpelly’s comments on this table: 
This is the broader history. Orthoclase is here, as in the pseudo-amygdaloid, of 
sporadic occurrence, and a product of the prehnite. 
The changes under II. may affect only the amygdules, or, if the matrix was preh- 
nitized, it applies to the whole mass of the amygdaloid. It does this in such a manner 
1R, Pumpelly, Geology of Wisconsin, Vol. III, p. 31. 
