522 LINGULA-FLAGS AND IGNEOUS ROCKS OF DOLGELLY. [Aug. 1 896,, 



Plate XXIY. 



Geological Sketch-map of the Neighbourhood of Dolgelly, on the 

 scale of 3 inches to the mile. 



Discussion. 



The President said that it was interesting to find that andesitio 

 lavas occurred in the Dolgelly Beds. Acid lavas were frequently- 

 found in the Lingula-Fl&gs of North and South Wales, and beds 

 mainly composed of volcanic ash occurred at the top of the Menevian 

 group at St. David's and near Maentwrog. He thought that most 

 of the diabase-intrusions in the area were of Ordovician age. 



Mr. W. W. Watts expressed the pleasure he felt that the Authors 

 were not content with proving that the diabase-masses were of 

 laccolitic character, though they had done that conclusively, but had 

 gone beyond and used the existence of the laccolite to draw further 

 conclusions as to the structure of the area. He reminded the Society 

 that he had described laccolites of similar diabase which occurred in 

 the Shelve district, resting unconformably on Ordovician strata and 

 covered by Silurian rocks which were altered at the contact. It 

 could be proved in Shropshire and Montgomeryshire that the 

 diabases had nothing in common with Carboniferous and post- 

 Carboniferous intrusions, and he therefore thought that the Authors' 

 suggestion with regard to the Dolgelly masses was founded on toe 

 slender evidence. 



Mr. Lake, in reply, said that the Authors were by no means 

 wedded to the hypothesis that the diabases were of post-Car- 

 boniferous age. The evidence derived from the faults is very- 

 uncertain. Parallel faults are not necessarily of the same age; 

 and even in the case of a single fault, movement may take place at 

 different periods. The absence of olivine tends rather to suggest 

 that the intrusions may be pre-Carboniferous ; and the question 

 must still remain an open one. 



