1866.] J". GEIKIE CAERICK, AYRSHIRE. 531 



instance, at least, there can be little doubt that the metamorphism 

 ought to be assigned to hydrothermal action. The very partial 

 manner in which the strata have been affected — the frequent occur- 

 rence of unaltered areas among crystalline rocks and vice versa — the 

 various degrees of intensity which characterize the metamorphism, 

 even when the beds undergoing change have much the same compo- 

 sition — all seem to point to the partial distribution of moisture or 

 water in the strata at the time metamorphic action ensued, so that 

 when heat began to attack the beds, its influence was aided in a 

 greater or less degree by the amount of water present in the rocks. 

 The probable source of this water will be considered presently. 



Many portions of the strata have merely undergone a process of 

 hardening, which has sometimes given to the beds a semicrystal- 

 line texture — a change which might be brought about without fusion 

 or softening. But the appearances presented in some places require 

 us to infer a former pasty or almost semifluid condition. It is un- 

 necessary to recapitulate the evidence on this head ; but reference 

 may be made to a few of the facts already adduced. 



The phenomena connected with isolated amygdaloidal areas are 

 especially worthy of study. The little cells have attained their 

 spherical shape at a time when the matrix had a certain plasticity. 

 If this plasticity had been caused by dry heat, it is diflicult to 

 understand how the granular portions of the same beds should have 

 escaped change, especially as their composition does not differ from 

 that of the amygdaloidal areas. The whole series of phenomena 

 associated with the gradual increase of separate amygdaloidal areas, 

 and the progressive formation in this Avay of spheroidal and colum- 

 nar amygdaloid, appear explicable only on the assumption of hydro- 

 thermal action. 



The appearances presented by felspar-porphyry seem, to bear out 

 the same view. Nothing is more striking than the often close 

 association of highly metamorphic with comparatively unaltered 

 areas. Greenish granular wacke has been observed so close to fel- 

 spar-porphyry that only a thin irregular ribbon of rudely schistose 

 serpentinous matter kept the two rocks from touching. It was in 

 this wacke that isolated vesicidar areas were first detected ; so that, 

 although the beds retain in great measure their granular structure, 

 yet they cannot be said to be quite unaltered. But that they should 

 have experienced so little change in the immediate vicinity of a 

 rock the nature of which evinces great intensity of metamorphic 

 action is not a little surprising. It is not improbable, however, 

 that the felspar-porphyry, having been reduced to a soft or viscous 

 state, has been partially forced out of its position by the weight of 

 superincumbent strata pressing upon it, and in this way intruded 

 among other rocks which had not undergone the same degree of 

 change. Of the truly metamorphic origin of the felspar-porphyry 

 there can be no doubt. Its behaviour at other points has been 

 referred to above. 



Of the stones of the brecciiform rocks or altered conglomerates, it 

 was remarked that they are for the most part of angular and sub- 



