432 Correspondence — Professor T. G. Bonnet/. 



questions bad prior claims, I have never been able to investigate it 

 systematically. And it is one beset with difficulties, especially 

 when we are dealing with rather old rocks, not the least being that 

 of determining when the alteration for which we have to account, 

 has taken place. But I have always tried, so far as was possible, 

 to call attention to any mineral changes which seemed distinctive, 

 and have sometimes ventured to suggest a cause for them. The 

 subject would well repay any young geologist with sufficient leisure, 

 and on this account, with the same motive as Mr. Merrill, I will 

 venture to offer a few remarks suggested by his paper. As regards 

 ' serpentinization ' I have come to the same conclusion as himself, 

 viz., that the change is deep-seated rather than superficial, and that 

 the same is true of certain other hydrous minerals, though it may 

 not hold with some at least of the zeolites. But the anomalies 

 which mineral change presents are apparently considerable. As 

 an example we may select the conversion of augite into hornblende. 

 Sometimes a crystalline grain of the former mineral may pass into- 

 one of the latter by a change seemingly no more than paramorphic ; 

 sometimes it may break up into a group of smaller grains, not 

 retaining exactly the original outline. Sometimes, carrying this 

 further, it may form one or more elongated crj^stals (this, as I have 

 pointed out, seems connected with pressure) ; and sometimes it may 

 be converted into a more or less fibrous and matted mass, in which, 

 case not seldom some chemical alteration seems to have taken place. 

 Again, as anotlier anomaly, some light-coloured augites seem very 

 stable, as in certain picrites and granites; while others, as in some 

 hemithrenes, change rather readily into serpentine. But in a mica- 

 trap the augite, probably also not rich in iron, seems to be very 

 unstable, not, however, altering to hornblende. In one dolerite the 

 augite seems to change more readily than the felspar, in another- 

 the reverse. Felspar sometimes changes into quartz and white 

 mica, sometimes undergoes other alterations. Here, as in some- 

 other cases, much probably depends upon whether the water present 

 is 'stagnant' or in motion. It would be easy to continue the list, 

 but this may suffice to give some idea of the problems presented, 

 and when they have been solved we may proceed to those presented' 

 by the devitrification of glassy igneous rocks. Suffice it to say that 

 the changes in a mineral seem to be the outcome of two separate 

 causes ; one, its chemical composition ; the other, its environment, 

 the principal factors in the latter being water, pressure, and heat. 

 The best way of attacking the problems will be by first studying 

 the simpler cases, viz., those in which we can be reasonably certain 

 that one factor in the environment has so far dominated over the 

 others, that they may be neglected ; and then proceeding to the more 

 complicated cases. Of this I think we may be certain, that in nature, - 

 with the same materials, similar causes produce identical results,, 

 and dissimilar causes the contrary. 



T. G. Bonnet. 



