522 PROCEEDIXGS OP THE GEOLOGICAL SOCIETY. [JuXlC 6, 



daloids, and porphyries have been produced. Eegarding* the origin 

 of the isolated amygdaloidal areas two opinions may be held. We 

 may suppose the cavities to have been formed by gases during a 

 pasty condition of the rocks, and the amygdaloidal minerals to have 

 been subsequently introduced. In this case the origin of the vesicles 

 would be entirely analogous to that of the cavities in lava. But 

 evidence is not wanting w^hich seems to lead to the inference that 

 the vesicular nature of the altered wackes may be due to segregation, 

 the excess of alkaline matter having separated from the felspathic 

 matrix to store itself up in little globules. [The same kind of 

 phenomena may be seen in many trap-rocks. Globules of quartz, 

 which are clearly not due to infiltration, abound in some felstones, 

 and they have also been detected in other igneous rocks.] Towards 

 the centre of an amygdaloidal area the vesicles are largest, and from 

 the centre outwards they gradually decrease in size, until they be- 

 come so small as to require a lens to distinguish them. In this 

 manner amygdaloidal areas shade off into the unaltered granular 

 portions of a wacke. It would seem, therefore, that, as long as the 

 metamorphic action proceeded, the vesicles increased in size until 

 they had reached their maximum *. Towards the circumference of 

 an area, as the action extended, the vesicles, it may be supposed, 

 continued to expand by receiving fresh supplies of gaseous matter. 

 The metamorphism, however, having suddenly ceased, left its work 

 incomplete ; and thus we find the smallest cavities at the edge, and 

 the larger ones in the centre, of an amygdaloidal area. That vesicles, 

 however, may have been formed by the segregation of amyg- 

 daloidal minerals f is rendered probable by the fact that the matrix 

 in which they occur is commonly harder and more compact (in 

 other words less alkaline^ and of a darker colour than the sur- 

 rounding wacke. It has also been pointed out that the hard 

 non-alkaline beds with which the softer magnesio- calcareous 

 wackes are interstratified have remained comparatively unaltered. 

 So far as was seen they are never amygdaloidal. The num- 

 ber of globular cavities is always greatest where the rock 

 undergoing alteration is most highly alkaUne. Of course, it must 

 be admitted that these appearances are also explicable on the 

 assumption of the gaseous formation of the cavities ; for the same 

 action which could only impart a crystalline or semicrystalline 

 texture to less basic strata might reduce highly alkaline wackes to 

 a pasty condition in which gas-cavities would readily form. But, 

 although the cellular character of these metamorj^hic rocks is doubt- 

 less mainly due to the expansive power of imprisoned gas, yet the 

 occasional origin of the cavities by segregation must not be overlooked. 

 Future chemical analyses will enable us to clear up this point. 



^ When the cavities are very closely aggregated, they are usually smaller 

 than vrhen they occur in less numbers. 



t I refer here to cavities of magnesian and calcareous matter ; it has been 

 already stated that, associated with these minerals, zeolites and hard white felspar 

 also occur in some places. Whether (on the supposition that the cellular nature 

 of the rock is due to segregation) the zeolites and felspar are contemporaneous 

 with or have supplanted the alkaline minerals can only be conjectured. 



