OF ARTS AND SCIENCES. 309 



a definite limit at which this progressive change stops, and tliat is when 

 all the augite is changed to its green pseudomorph, and a large per- 

 centage of the rest of the rock consists of pseudo-amygdules of delessite, 

 and partial pseudomorphs of this after plagioclase. The occurrence of 

 epidote and quartz is not general, and is then confined to scattering 

 pseudo-amygdules, in which these minerals have succeeded prehnite, 

 perhaps in the local absence of the conditions necessary to produce the 

 usual delessite. 



Thus I conceive that the extent of the change to the pseudo-amygda- 

 loidal form is conditioned essentially by the amount of augite jiresent, 

 to supply first the lime necessary to aid in changing the plagioclase to 

 prehnite, and next the iron and magnesia to form the delessite, whether 

 by acting directly on the feldspar substance or on the prehnite. 



The amygdaloids proper were, probably, both structurally and chem- 

 ically, somewhat different from the lower zone, in that it is reasonable 

 to suppose that, in addition to being more or less porous, they contained 

 a greater or less amount of amorphous base, which is more easily altered 

 than a crystalline aggregate. But, from whatever cause, the amygda- 

 loids have, as we have seen, been capable of much greater changes than 

 the lower zone : in them the tendency is undoubtedly towards the for- 

 mation of quartz, chlorite, and epidote rocks as a more stable limit, 

 through the mediation of prehnite and calcite. 



There is one form of change in some amygdaloids that I have not 

 mentioned ; namely, the laumontitic. I have not had sufficiently good 

 matei'ial to work upon, and have not studied it to any extent under the 

 microscope. Wherever I have been able to determine its relative age, 

 as compared with prehnite, I have found it to be older than the latter. 

 The macroscopic appearance of some beds of this amygdaloid suggest, 

 that they have not only had the cavities filled with laumontite, but that 

 the matrix has been more or less changed to the same zeolite. 



