l^art 3] COMPOSITE sill at newtojs' abbot. 263 



contact-alteration ; in these rocks there were formerly found excellent large 

 specimens of Trimerocephalus. The average dip on the south of the summit 

 is 30^ to 35° south-south-eastwards, indicated by the junction of the spotted 

 slates, by the ' bostonite '-vein, and by the inclination of the junction at the 

 floor of the quarry. 



In the lower q-uany the upper surface dips steeply westwards at 

 60"^, altering the overljdng slates, in which Posidonomya venusta 

 is found ; the lower surface is beneath the Teign alluvium. North 

 of the summit the slope is steep, and it is possible that the dyke- 

 feeder of the sill is here ; but no satisfactory evidence could be 

 obtained to decide this point. 



III. Macroscopic Chaeactees. 



Upper Quarry. — Three rock-types are here in evidence — a 

 dark picrite, ordinary dolerite Aveathering a rusty brown, and pale 

 veins cutting the dolerite ; these are much lighter in colour, and 

 the ferromagnesian minerals are evidently present in but small 

 quantity. 



(«) The picrite is dark green, with black spots of olivine, and 

 the augite shows slight ' schiller ' effect ; the rock is hard and re- 

 markably fresh. Its most characteristic feature is the ' xenolithic ' 

 structure and a certain amount of veining ; the interspaces are 

 •occupied by a very decomposed brown material, weathering simi- 

 larly to the ordinary dolerite. Sections were made, but showed 

 only a structureless chloritic mass. The brecciated appearance is 

 due to these xenoliths, and the occurrence is very similar to that 

 described by Dr. A. Harker from the Skye peridotites ; the later 

 doleritic magma has partly resorbed the picrite, further proof of 

 which will be found in the discussion of the xenoliths in the 

 dolerite. 



A larger, greyish vein is formed of a decomposed clayey 

 material with fibres of tremolite ; this has also broken down, and a 

 few fibres only can be extracted whole. The exact relations of 

 this vein cannot be ascertained, as it disappears vertically down- 

 wards ; its formation may be due to subsequent movement along a 

 •doleritic vein in connexion with the slight thrusting already 

 mentioned or during the injection of the ' bostonite '-vein ; 

 tremolite is found in the picrite, but as an alteration -product of 

 the augite. 



(5) Xenoliths in the dolerite. — The picrite thins out rapidly, 

 and here the rock has been quarried for a few feet more nearly 

 along the strike ; but picrite is not visible. The dolerite weathers 

 uniformly, except for some isolated patches from which the 

 crust is easily removed, exposing a hard centre. At first sight, 

 this appears to be due to spheroidal weathering, but the junction 

 is fairly sharp, and microscopic examination confirms the presence 

 of xenoliths ; they appear to be restricted to a zone in close 

 -association with the salic vein. It will be seen later that tliese 



t2 



