A SUMMER RAMBLE AMONG THE HEBRIDES. 131 



On what principle, and under what direction, have results so 

 various taken place in vesicles of the same rock, that in many 

 instances occur scarce half an inch apart ? Why, for instance, 

 should that vesicle have elaborated only green earth, and 

 the vesicle separated from, it by a partition barely a line in 

 thickness, have elaborated only chalcedony 1 Why should this 

 chamber contain only a quartzose compound of oxygen and 

 silica, and that second chamber beside it contain only a cal- 

 careous compound of lime and carbonic acid 1 ? What law 

 directed infiltrations so diverse to seek out for themselves ve- 

 sicles in such close neighbourhood, and to keep, in so many 

 instances, each to its own vesicle ? I can but state the prob- 

 lem, not solve it. The groupes of heliotropes clustered 

 each around its bulky centrical mass seem to show that the 

 principle of molecular attraction may be operative in very 

 dense mediae, in a hard amygdaloidal trap even ; and it 

 seems not improbable, that to this law, which draws atom 

 to its kindred atom, as clansmen of old used to speed at the 

 mustering signal to their gathering place, the various chemis- 

 try of the vesicles may owe its variety. 



I shall attempt stating the chemical problem furnished by 

 the vesicles here in a mechanical form. Let us suppose that 

 every vesicle was a chamber furnished with a door, and that 

 beside eveiy door there watched, as in the draught doors of 

 our coal-pits, some one to open and shut it, as circumstances 

 might require. Let us suppose further, that for a certain 

 time an infusion of green earth pervaded the surrounding 

 mass, and percolated through it, and that every door was open- 

 ed to receive a portion of the infusion. We find that no ve- 

 sicle wants its coating of this earthy mineral. The coating 

 received, however, one-half the doors shut, while the other 

 half remained agap, and filled with green earth entirely. 

 Next followed a series of alternate infusions of chalcedony, 

 jasper, and quartz ; many doors opened and received some 



