206 



MR. H. H. THOMAS ON TILE 



[May 19 1 1,. 



than the older basic and intermediate rocks, but the vesicles are 

 larger and not confined to the margins of the sheets. They often 

 immediately succeed beds of red clay, or some other sediment which 

 shows no noticeable induration at the contact. 



Although their intrusive nature is open to doubt, I am led to 

 regard these dolerites as intrusions on the following grounds :• — ■ 

 (1) their limited occurrence and striking dissimilarity to all the 

 other rocks of the series ; and (2) their thickness and their 

 indiscriminate association with rocks of widely different types. 

 The association of dolerite sheets with beds of red clay and the 

 absence of any important metamorphic effect are points commented 

 upon by Mr. Harker in connexion with the basic sills of the Western 

 Isles of Scotland. 1 



Fig. 12. 



A=Typical ophitic dolerite of the Skomer Series ; below ike acid series of the 

 Table (Skomer). It rests upon a bed of red and green mudstone, 2 feet 

 thick. Slide E 7118, ordinary light. X 25 diameters. 



B = Coarse ophitic dolerite; from the Anvil, the south-western point of the 

 Wooltack peninsula. Slide E 7755, ordinary light. X 25 diameters. 



The dolerite of Anvil Point is regarded as a lava, for it is 

 succeeded by a bed of red clay which has evidently been formed in 

 part from the igneous rock beneath. 



Under the microscope the Skomer dolerites are finely crystalline 

 rocks, consisting of labradorite laths, similar to, but slightly more 

 basic than, those of the basalts, sometimes with a tendency to 

 stellate grouping, enclosed ophitically by clots of purplish augite- 



1 'Small Isles of Inverness' Mem. Geol. Surv. Scotl. 1908, p. 59. 



