1866.] J. GEIKIE — CAEEICK, AYRSHIRE. 533 



Assuming that the metamorphism of these strata has been mainly 

 effected by hydrothermal action, a word or two may be added re- 

 garding the probable source from which the water has been derived. 

 AH rocks are found to contain a variable amount of water ; but the 

 strata nearest the surface of the earth are, for the most part, better 

 saturated with moisture than the beds at lower depths. The supply, 

 when derived from rains and the water of springs, rivers, and lakes, 

 must from various causes be very partially distributed. If, there- 

 fore,, the intensity of metamorphic change be influenced by the 

 amount of water present in the rocks, we should expect to find 

 the strata often showing sporadic areas of alterations. Nor need we 

 be surprised when metamorphic beds occur superimposed upon less 

 metamorphosed and unaltered rocks. "With regard to wide regional 

 metamorphism, like that of Canada, the Scottish Highlands, and 

 Norway, it is probable that the strata acquired their metamorphic 

 character while they lay underneath the bottom of the sea. During 

 such a period or periods of submergence, water could not fail 

 to find its way into the subjacent rocks, down through which it 

 would continue to percolate until it reached a point where the con- 

 ditions of heat might enable it to attack the strata and gradually 

 effect their metamorphism. The action of the heated water would 

 doubtless be often aided by the chemical reagents it held in solution, 

 and the ultimate character of a metamorphic rock might not infre- 

 quently depend upon the nature of such acid and saline solutions. 

 But the present aspect of the Carrick metamorphic rocks, at all 

 events, is certainly not due to this cause alone, but chiefly to the ori- 

 ginal composition of the unaltered strata. The elements necessary to 

 the formation of diorites and felstones were not introduced by water 

 during the process of alteration, but already existed in the beds 

 before metamorphic action began. 



On the hypothesis that the water necessary to hydrothermal 

 action has been supplied in the manner indicated, we can under- 

 stand how the lower beds of the Silurians of Carrick have escaped 

 alteration ; for either the water percolating downwards never pene- 

 trated so far, or else was present in too small quantity to induce a 

 wide-spread change. It is quite consistent with the views sup- 

 ported above, to suppose that during its downward passage the 

 water may have been deprived of certain chemical reagents before it 

 reached great depths, so as in some measure to have become 

 weakened, and thus less capable of affecting the condition of the 

 strata. When its passage, however, was aided by jointing and 

 fractures, it might occasionally sink to great depths, and there give 

 rise to metamorphic action. In this manner, we may account for 

 those isolated areas of crystalline rock that occur among the un- 

 altered strata, on a lower horizon than the chief metamorphic 

 masses. 



The only igneous rocks of the district are a few dykes of felstone 

 and greenstone, which are evidently of much later date than the 

 metamorphism. The granular wackes through which they pass 

 usually remain quite unaltered, even at their immediate junction with 



