510 TEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 6, 



duced. Serpentine does not, however, seem to have been the first 

 substance which occupied these cavities. The serpentine, on exposure 

 to atmospheric influence, frequently becomes much weathered, and 

 sometimes entirely disappears, leaving a crystalline skeleton in the 

 form of either tremolite, asbestos, or some other nearly allied mineral. 

 Asbestos is often found lining the sides of the limestone-cavities in 

 decomposed specimens of serpentinous limestones on the east side of 

 Derryclare Lake. This mineral is here so arranged as to present 

 the fine shell-structure alluded to by Dr. Carpenter as forming the 

 proper walls of the chambers of the Eozoon Canadense, and which 

 he speaks of as being '' made up of a multitude of extremely delicate 

 acicuK standing side by side like the fibres of asbestos." 



Specimens of the serpentinous limestones from Lisoughter often ex- 

 hibit in some parts only the tremolitic skeleton, the serpentine proper 

 having disappeared. Specimens also occur in which portions of the 

 serpentine remain intact, while other portions gradually shade off 

 to the condition of tremolite. 



The complex nature of serpentine has been frequently alluded to 

 by mineralogists, and also its varying composition. These circum- 

 stances result from the variable amount and the variable nature of the 

 skeleton upon which the hydrous silicate of magnesia has been depo- 

 sited. In the case of the serpentinous limestone of Connemara, there is 

 good reason for inferring that the tremolitic and asbestose skeletons 

 were formed at the period when metamorphic action was changing 

 the purely sedimentary rocks. One of the most abundant minerals 

 produced by this metamorphic action was hornblende, a substance 

 which enters largely into the composition of the gneissose rocks 

 superposed on the limestones; and the tremolitic variety of this 

 mineral appears to have found its way into the cavities of the con- 

 torted limestones. 



The following is the composition of tremolite from Fahlun : — 



Silica 60-10 



Magnesia 24-31 



Lime 12-73 



Protoxide of iron variable. 



tremolite being a silicate of magnesia and lime. The serpentine of 

 Ballynahinch has the following composition : — 



Silica 40-12 



Magnesia 40-04 



Afumina 200 



Protoxide of iron 3-47 



Water 13.36 



98-99 

 The occurrence of serpentine in connexion with metamorphic 

 limestones is a common circumstance. It is found in association 

 with the limestones of Glen Tilt ; and here, as at Connemara, tremo- 

 lite is also an accompanying mineral. 



The supposed organic portions of the serpentinous limestones of 

 Connemara do not result. from animal structure, but purely from 

 mineral association. Had fossils of any kind presented themselves 

 in the limestones of this district, they ought to have occurred in 



