78 Lamination of Volcanic Rocks part i. 



parallel planes. 1 These allied forces, therefore, have 

 played an important part in the lamination of the mass, 

 but they cannot be considered the primary force ; for 

 the several kinds of nodules, both the smallest and the 

 largest, are internally zoned with excessively fine shades 

 of colour, parallel to the lamination of the whole ; and 

 many of them are, also, externally marked in the same 

 direction with parallel ridges and furrows, which have 

 not been produced by weathering. 



Some of the finest streaks of colour in the stony 

 layers, alternating with the obsidian, can be distinctly 

 seen to be due to an incipient crystallisation of the 

 constituent minerals. The extent to which the minerals 

 have crystallised can, also, be distinctly seen to be 

 connected with the greater or less size, and with the 

 number, of the minute, flatteued, crenulated air-cavities 

 or fissures. Numerous facts, as in the case of geodes, 

 and of cavities in silicified wood, in primary rocks, and 

 in veins, show that crystallisation is much favoured 

 by space. Hence, I conclude, that, if in a mass of 

 cooling volcanic rock, any cause produced in parallel 

 planes a number of minute fissures or zones of less 

 tension, (which from the pent-up vapours would often 

 be expanded into crenulated air-cavities), the crystal- 

 lisation of the constituent parts, and probably the for- 

 mation of concretions, would be superinduced or much 

 favoured in such planes ; and thus, a laminated struc- 

 ture of the kind we are here considering would be 

 generated. 



That some cause does produce parallel zones of less 



1 The formation, indeed, of a large crystal of any mineral in a 

 rock of mixed composition implies an aggregation of the requisite 

 atoms, allied to concretionary action. The cause of the cryscals of 

 feldspar in these rocks of Ascension, being all placed lengthways, is 

 probably the same with that which elongates and flattens all the 

 brown sphaerulitic globules (which behave like feldspir under the 

 blowpipe) in this same direction. 



