GABBROS ETC. IN SCOTLAND AND IRELAND. 81 



we are now describing arises from the circumstance that we are able 

 to study in them the incipient stages of the several transformations 

 which their minerals undergo. From examples in which the 

 minerals are absolutely fresh and unaltered, we may trace every 

 gradation into the most highly altered forms. All those alterations 

 of minerals, which have been so well traced in the case of the older 

 roclvs by Mr. Allport* and Professor Bonney, may be as clearly, if 

 not as frequently, found exemplified in the case of these rocks of 

 Tertiary age. 



There is one point which comes out from these studies which 

 must be carefully borne in mind. The same mineral species does 

 not always exhibit the same degree of chemical instability or liability 

 to be acted upon by chemical agents. Eor this we might perhaps 

 be prepared by the circumstance that the behaviour of different 

 varieties of the same mineral species with artificial solvents also 

 varies in a very remarkable manner. Thus different varieties of 

 olivine have been shown to differ in the most striking manner in their 

 solubility in acids. Perhaps the most common rule is that the olivine 

 is the most easily altered constituent in the basic rocks, then 

 follows augite, and lastly felspar f. But there are cases in which 

 the rock appears to contain an abnormally unstable augite or 

 felspar, and this sequence of decomposition is no longer maintained. 

 There are even some picrites, like those of Central Rum, in which 

 the adventitious crystals of anorthite appear to be of such a singu- 

 larly unstable character that they have been converted into zeolites 

 and other decomposition-products, while the augites, enstatites, and 

 olivines show no sign of change. Even in the same mineral in a 

 mass of rock, among crystals which exhibit no recognizable difference, 

 we may find some undergoing extreme alteration, while others 

 around them remain quite unchanged. What are the subtle causes 

 of the different degrees of susceptibility to the same chemical forces 

 in different crystals of the same mineral, it is, in our present state 

 of knowledge, impossible even to suggest. 



The alterations which the minerals of these rocks undergo are of 

 the most varied kind, and are evidently due to the action of different 

 causes, among which the five following may perhaps be reckoned as 

 the chief: — 



§ 1. Action of surrounding Magma upon Crystals. 



The progressive acidifying of the glassy base of a rock by the 

 crystallizing of its more basic constituents is a characteristic of the 

 acid and intermediate classes of rocks, rather than of the basic ones. 

 Hence it is in the former rather than in the latter that we notice 

 those very conspicuous changes of the exterior portions of crystals 

 where they have been in contact with the enveloping acid material. 



* See Quart. Journ. Geol. Soc. toI. xxx. (1874) pp. 529-567. 



t Professor Bonney, who has paid much attention to this question, however, 

 informs me that, according to his experience, among igneous rocks of different 

 areas, the felspars, as a general rule, succumb to weathering-action before the 

 augites. 



Q. J. G. S. No. 165. e 



