532 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [JuHG 6, 



angular shapes, but often with a general approach to roundness, 

 especially when imbedded in a matrix. The whole aspect of these 

 stones forcibly suggests hydrothermal action. In many places the 

 pebbles have been mutually squeezed, so as to become distorted, and 

 sometimes to assume hexagonal forms, thus implying a somewhat 

 softened condition*. 



Without going into more detail, allusion may be made to an inter- 

 esting point in connexion with certain metamorphic rocks of Lower 

 Old Eed age, belonging to another district of Ayrshire, situated 

 some miles to the north-east of Girvan, on which the Survey 

 was engaged during the past year. The rocks of this district are 

 chiefly felspathic varieties, and present very much the same phe- 

 nomena as has been shown to characterize the altered felspathic 

 wackes of the Silurian strata. Near the farm of Knockdon, on the 

 Water of Girvan, occur conglomerates and felspathic sandstones 

 which have been converted in places into dark and light pink fel- 

 stones. Many portions of these felstones were found to be porphy- 

 ritic, often abundantly so, with fan-like radiating fibrous crystals of 

 a dark greenish zeolite. Sometimes the same mineral occurred in 

 amorphous blotches. These zeolites did not fill up amygdaloidal 

 cavities, but were in every way analogous to the porphyritic crystals 

 of a felspar-porphyry. We must therefore admit that, in some way 

 or other, water has had to do with the production of these meta- 

 morphic felstones. 



The nature and origin of the veins which traverse the metamorphic 

 Silurian rocks of Carrick aff'ord very strong sujDport to this con- 

 clusion. I cannot enter here into the evidence which goes to prove 

 that the thick ramifying veins of carbonate of lime, diallage, felspar, 

 zeolite, (fee, are all of the same age as the metamorphism, and are not 

 due to subsequent infiltration. Yeins of carbonate of lime have been 

 described as proceeding from imbedded blocks of limestone ; knotted 

 strings of diallage crystals have been pointed out as forming part of the 

 bed in which they occur ; veins of felspar are associated with ex- 

 ceedingly coarse diorite in such a way as to show that their origin 

 must be contemporaneous with that of the diorite ; while abundant 

 veins of a beautiful white fibrous zeolite, that traverse the strata in 

 many places, are also to be assigned to the date of the general 

 metamorphism. Mr. Eichard Smith, of the Geological Museum, 

 Jermyn Street, kindly undertook the analysis of this zeolite, the 

 composition of which he ascertained to correspond with that of 

 pectolitef. 



^ But such distorted stones must be carefully distinguished from the appear- 

 ance presented by a fine-grained semicrystahine felspathic rock which seems to 

 be entirely made up of little rounded pellets like pebbles. This, however, is a 

 superinduced structure. The pellets vary from the size of peas to that of hazel- 

 nuts. When they press closely, they frequently become five- or six-sided. It is 

 difficult to account for this peculiar structure ; but that it has been induced by 

 hydrothermal action, and not by dry heat, is borne out by the testimony of the 

 surrounding metamorphic phenomena. 



f Since Mr. Smith's analysis was made, I find that pectolite had been 

 obtained by Prof, James Nicol from the same neighbourhood. 



