Bocks vedr Heathcote. 333 



recrystallization of material already present in a rock, which is 

 the normal effect produced in a sediment by an igneous intru- 

 sion. The ordinary minerals produced by contact metamorphism 

 aro quite unrepresented. In place of this there has been a 

 fundamental change in the chemical composition of the rock. 

 The original diabasic oonstituents — lime, magnesiai and iron — - 

 have been more or less completely removed, and replaced by 

 chalcedonic silica. It seems to me quite improbable that a 

 magma of the composition of diabase should be capable of sup- 

 plying to an invaded rock silica in such large quantities. This 

 vie^v, moreover, can only be maintained on the hj'pothesis that 

 the diabase is intrusive. I have stated above my reasons for 

 regarding the bulk of the diabase as consisting of lavas and 

 pyroelastic rocks, and the cherts as being probably silicified 

 bedded submarine diabase tuffs, or at any rate fragmental 

 diabase rocks. On this view the diabase may be older than 

 the cherts, but is more probably practically contemporaneous 

 with them, and therefore cannot be regarded as the direct agent 

 of chemical change. 



The exi>lanation which I offer of the production of the cherts 

 and cherty rocks is that they are the result of metasoinatic re- 

 placement in certain parts of the Ordoviciaiis by silica-bearing 

 solutions traversing the rocks subsequent to the formation of the 

 diabase and the Ordovicians in contact with it. This view 

 receives confirmation from the fact that the diabase, like the 

 Ordovicians, is locally silicified, and in places almost completely 

 replaced by silica, as will be described below. This circumstance 

 makes it improbable that the diabase, which is itself silicified, 

 can be the direct cause of silicification of the cherty rocks. 



The limitation of the silicification to those rocks near the 

 junction with the diabase is a noteworthy feature, and must, I 

 think, bo discussed in connection with the original composition 

 of these rocks. These I have shown to be mainly composed of 

 fragmental diabasio material near the diabase, and as you go 

 westwards from the diabase junction you pass into higher beds 

 in the series, which gi-adually take on the characters of normal 

 shales. I think, then, that solutions carrying silica traversed 

 diabase and Ordovician alike, and that selective silicification 

 took place ; some of the diabases, and the Ordovicians which 



