158 
THE CAMBRIAN VOLCANOES 
BOOK III 
stratigrapliical order.^ We thus learn that the eruptions of this district 
came to an end in an early part of the Cambrian period. But as we shall 
see in the following pages, volcanic activity was subsequently renewed at no 
great distance in the next or Silurian period. 
' Dr. Hicks kas noted the occurrence of “ volcanic tuff” in the Lower Lingula Flags of Porth- 
y-Rhaw, a little to the east of St. David’s [Quart. Journ, Geol. Soc. vol. xx. 1864, p. 240). This 
intercalation is marked as a “dyke” in the MS., notes of Sir A. Q. Ramsay on a copy of the 
Geological Survey map of the district. 
