Yol. 55.] GEOLOGY OF NORTHERN ANGLESEY. 6Q9 



Prof. Blake has shown that part of the Llanbadrig limestone is 

 oolitic/ with compound grains : that is to say, two or three grains 

 are often enclosed within a larger grain. Erom this structure, as 

 well as from its intrusive appearance in the mass, he deduces that 

 it is of later date than the rocks among which it occurs and that it 

 was formed by the action of calcareous springs. I did not find an 

 example of this structure at Llanbadrig, though I do not doubt its 

 occurrence there ; but there is a band of light-brown limestone on 

 the shore of Cemaes Bay, along the course of the rifle-range, which 

 displays similar structures, except that the oolitic grains are not 

 compound. This oolitic structure shows some breaking-down by 

 veining, recrystallization, and a certain amount of dolomitization 

 (N.A. 87), and may once have been a more common feature of 

 these rocks.^ The nuclei show no trace of organisms. 



No fossils have yet been discovered in the limestones, though 

 there are some structures in a microscope-slide (JST.A. 12) from the 

 Llanbadrig limestone which may possibly be organic. The general 

 absence of fossils fits in with Prof, Blake's chemical theory, but if 

 we consider the reconstruction that these rocks have undergone by 

 shattering, crystallization, veining, dolomitization, etc., we should 

 expect very few organic remains that the limestones may have once 

 contained to escape destruction. The crush- con glomerate explana- 

 tion of the ' agglomerates ' of the north coast should clear away the 

 difficulty experienced by Prof. Blake in accepting the limestones as 

 ordinary bedded rocks. Their irregular, and sometimes even intrusive,, 

 appearance is an effect due to these hard resistant masses having 

 been broken and crushed in by complicated movements among 

 thinner strata. 



(b) Quart zites and ^ Quartz- Knobs .^ 



The bands and pieces of quartzite which have been mentioned 

 in connexion with the crush-conglomerates are for the most part 

 evidently altered grits and sandstones. There are, however, both 

 in and away from the crush-zones, thicker masses of quartz-rock of 

 which Prof. Blake speaks as presenting ' the most remarkable features 

 of all the Anglesey rocks.' ^ He refuses to apply to these the name of 

 quartzite, styling them ' quartz-knobs,' and suggests that they have 

 been produced by hot siliceous springs which may or may not have 

 reached the surface. He distinguishes them by their purity of com- 

 position, their internal structure, obscurity of bedding, and form of 

 outcrop, their characteristic mode of occurrence being as isolated 

 hummocks which vary in size from a low mound to a good-sized hill. 

 In composition they are certainly of remarkable purity ; for instance, 

 it was stated to me that the Craig Wen rock had yielded on analysis 



^ Quart. Journ. Geol. Soc. vol. xliv (1888) p. 518; 'Microsc. Struct, of 

 Older Rocks of Anglesey,' Brit. Assoc. Rep. 1888 (Bath) p. o89. 



^ It may be remarked that oolitic limestones are known in the Dalradian of 

 Scotland ; see A. Geikie, ' Text-Book of Geol' 3rd ed. (1893) p. 150, footnote. 



^ Quart. Journ. Geol. Soc. vol. xliv (1888) p. 475. See also Brit. Assoc. 

 Eep. 1888 (Bath) pp. 390-392. 



