114 Scientific Proceedings, Royal Dublin Society. 



But the calcite veins and aggregates play a far smaller part, even 

 in the breeciated varieties of the rock, than would at first appear ; 

 for films of calcite have arisen along all the joint-planes and 

 irregular surfaces of division in the decomposing lavas, so that 

 solid lumps may be picked out, apparently of limestone, but which 

 are in reality small joint-blocks coated completely by the calcium 

 carbonate. The mass of the diabase effervesces only feebly 

 with cold nitric acid, and with little more briskness in hot acid, 

 the bubbles even then arising from local cracks, and not from the 

 rock-fragments as a whole. It is, perhaps, remarkable that so 

 much calcite, and so little dolomite, seems to have arisen in these 

 lavas, rich as they are in decomposing magnesian silicates. 



A more remarkable change which has taken place in the 

 compact diabase has resulted in a true pseudomorphosis. Silica, 

 as Prof. Blake has noted, has permeated the rock, and the purple 

 colour mentioned by him is, as far as my observations go, connected 

 far more often with this chalcedonic replacement than with the 

 calcareous infiltrations. ■ The calcite masses in the breccias are, 

 however, often pink. This silica, which one would certainly 

 associate with the action of hot springs, rising, perhaps, in pre- 

 Cambrian times at the close of local volcanic activity, has actually 

 replaced the igneous rock by a compact red or purple-red jasper. 

 This pseudomorphic action takes place here and there along lines 

 and veins, but often extends to the whole interior of one of the 

 spheroidal masses of the diabase. The original crust of spherulitic 

 tachylyte, now altered to variolite, has resisted this attack ; hence 

 we find the films of ancient glass wrapping round masses of red 

 jasper, which latter are full of shrinkage- cracks and irregular 

 little hollows. All stages of the pseudomorphosis may occur ; but 

 much of the material can no longer be scratched by the knife, and 

 includes only little scattered relics of the original gray-green 

 diabase. 



While in places the jasper passes into a clearer red quartz-rock, 

 which has a specific gravity of 2*63, in great part it is full of 

 ferric decomposition-products, which render it opaque even in thin 

 sections. Thus one specimen, in which the flow- structure of the 

 original lava has been preserved, has a specific gravity as high as 

 3*13, considerably in excess of that of the diabase in its more 

 normal phase of alteration. 



