1866.] ■ J. GEIKIE — CAREICKj AYRSHIRE. 529 



Of course it may be said that this magnesian character has been, 

 superinduced by metamorphic action. Perhaps it has ; and if the 

 process of alteration had been continued, we might have had 

 serpentine in place of an impure magnesian limestone. "We must 

 suppose that metamorphism is a gradual operation, and that the 

 rocks acted npon assume various characters as the action proceeds. 

 Serpentine is evidently an advanced stage in the process. 



3. Association of Diorite and Serjpentine. — The junction between 

 serpentine and diorite is often very distinct ; but in. other places 

 where the latter is fine-grained and highly magnesian, it passes so 

 insensibly into the former that we are frequently at a loss to tell at 

 what point we must cease to call the rock a diorite, and when we 

 ought to begin to describe it as serpentine^. In cases of this kind 

 we may reasonably infer that the original composition of the beds 

 which has given rise to the diorites shaded gradually into that of 

 those more highly magnesian beds whose metamorphism has re- 

 sulted in serpentine. On the other hand, when the junction between 

 that rock and diorite is well marked, it appears probable that the 

 composition of the various strata before metamorphic action began 

 was also well marked, one bed not shading off into another as in 

 the previous case. 



A few notes on the better-marked junctions of serpentine with 

 diorite may be interesting. As it approaches diorite, serpentine 

 frequently becomes rudely schistose and sometimes quartzose — so 

 much so that it might occasionally be described as a highly silicated 

 felspathic schist, tinged green with magnesian matter. Here, also, 

 it sometimes shows irregular interrupted bands of yellow, green, 

 and brown Lydian-stones. The junction with the diorite is very 

 irregular, the serpentine appearing as if it had been caught up 

 every here and there, while occasionally long branching fingers of 

 magnesian matter are protruded into the diorite. Small veins of 

 diorite, also, in some cases pass into the serpentine. It is, no 

 doubt, appearances of this kind which in some cases have led geolo- 

 gists to consider serpentine as of igneous origin. On the sea-shore 

 at Lendalfoot many excellent junctions of the nature alluded to may 

 be studied. The strata at this place are very much confused; 

 nevertheless alternations of serpentine with diorites and felstones 

 succeed each other in such a way as to indicate that we have here 

 the original bedding. 



Y. Limestone. 



It now only remains to add a few words about the limestones f 



^ On the hills a little to the north-east of the village of Colmonell (river 

 Stinchar) a very peculiar serpentine occurs. Interspersed throughout a base 

 of dark-green serpentine (which in places is porphyritic with bronzite) abun- 

 dant granules and interrupted nodular threads of white felspar make their 

 appearance. The matrix in which this felspar occurs is harder than the other 

 -portions of the rock, where the serpentine presents its normal character. As 

 this peculiar rock certainly passes into diorite (hornblende and felspar) there can 

 be no doubt that the hardening of the green serpentinous matrix is due to the 

 presence of lime in chemical union with the silicate of magnesia. 



t These limestone^ have yielded fossils of Lower Silurian age, a list of which 



