ASSOCIATED KOCKS OF THE BKEIDDEX HILLS. 545 



this one. Where thrust in thin dykes into the shales, large felspars 

 are still visible, but they are set in a microcrystalline matrix com- 

 posed of small felspar crystals and calcite, which latter has taken the 

 place of all but the felspar, and even attacked that. This rock is 

 either an eustatite or olivine diabase. 



These rocks are all intrusive into the Criggion shales surrounding 

 them, and evidence of this is abundant throughout the district. On a 

 small branch of Belleisle Brook the subjoined section (fig. 5) shows the 



Fig. 5. — Section in small trihutary of Belleisle Brook. 

 (Length 15 feet.) 



D. Diabase intrusive in shale, S'. 



S". Fragment of shale caught in diabase. 



intrusion of diabase, and at least two other distinct veins of it have 

 been found in other parts of this brook and its tributaries, connecting 

 the Brimford Wood with the northern massifs. Then, again, the 

 actual contact is seen to the west of the Criggan, where the diabase 

 alters the shales very considerably at contact. A thin section of 

 shale taken from this spot shows a development of small scales of 

 white mica and of greenish brown dichroic mica parallel to the 

 lamination ; but the most interesting feature is the incipient breccia- 

 tion of the rock by a number of minute faults evidently caused by 

 the violence of intrusion. The faults vary in throw from -2 to -03 

 inch, and some of the very smallest may be as little as '006 inch in 

 throw ; most of them are normal, but the largest and a few others 

 are reversed — I count 14 of them in a specimen 2 inches long. On 

 the S.E. side similar hardening takes place, the shales often being 

 burnt red ; but I cannot recognize any secondary crystallization. At 

 Trewem and round Eodney's Pillar Hill the shales are also much 

 contorted and hardened near the junction ; in the quarry near But- 

 tington station sand-beds are converted into quartzite. 



As to the age of the diabases, we cannot do much more than con- 

 jecture. In no place do they actually traverse Silurian rocks, but 

 near Buttington the later rocks are as remarkably distui"bed as the 

 Cambrian, and I trace this disturbance to the intrusion of igneous rock. 

 If they are really post-Silurian it is very difficult to assign their 

 exact age ; for in this part of the country most of the post-Silurian 

 igneous rocks are later than early Carboniferous, and are dolerites 

 much less altered than these, and with a different mineral composition 

 and structure. It is of course possible that they may belong to the 



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