T. Mellard Beade — The Dimetkm of St. Davids. 559 



show that the rock is not in any sense Archeean, but is post-Cambrian 

 and intrusive. 



First, a few words with regard to the nature of the faults required 

 on Di-. Hicks's hj^pothesis to entitle the granite or granitoidite to be 

 considered the oldest of the Archtean series he has sought to establish 

 at St. Davids. 



The bedded rocks surrounding the granite boss have a high dip 

 often approaching the vertical. On the one side to the south and 

 in contact with it lie the admittedly Cambrian rocks, and on the other 

 to the north-west also in contact, the underlying volcanic series or 

 " Pebidiau " of Hicks. Now as the "Dimetian" (granite) is in Dr. 

 Hicks's view older than either series, yet occurs between them, 

 it cannot have been placed in its present position by the folding 

 which tilted the Cambrians and volcanics. It must therefore have 

 been pushed through these highly-inclined rocks as a sort of plug, 

 consequently Dr. Hicks in his map has shown the granite cut off 

 from the surrounding rocks by a set of boundary-faults. To establish 

 the existence of such a remarkable phenomenon would require the 

 most complete and reliable evidence — such evidence as I venture to 

 think has not yet been given. 



If we consider the throw required in this way to bring the 

 "Dimetian " into its present position, we shall find that it will have 

 to be estimated by thousands of feet. Does any geologist venture 

 to assert that the so-called junction faults represent any such a 

 movement? On the contrary, had such tremendous displacements 

 occurred, I hold that geologists would have universally recognized 

 them, whereas some dispute the very existence of the junction-faults. 

 Unless some one holding Dr. Hicks's view can frame a consistent 

 theory by which' the "Dimetian" can be got into its present position 

 without the aid of these great throws, I shall continue to hold this 

 reasoning unassailable. 



It may be said that these are mere general considerations that are 

 not conclusive, or at least do not carry conviction to some minds. 

 Let us see if there ai'e evidences of another class which support this 

 contention. 



In the much-examined junction of the granite and Cambrian con- 

 glomerate on the right bank of the Alan at Porth-clais, of which 

 there is a wood-cut in Dr. Hicks's paper ^ the granite shades off into 

 the conglomerate, here not much more than a coarse grit, in a way 

 to make their separate recognition difficult. The conglomerate in 

 consequence of a reversed dip lies upon the green Cambrian shales 

 or sandstones,- which dip under it and the granite to the north at an 

 angle of about 62°. 



The conglomerate is seen also above the granite, but probably this 

 is only the junction face disclosed through the granite having been 

 removed from the front of the conglomerate. To explain myself 



1 a.J.G.S. vol. xl. p. 532. 



^ The rock is very like the grauwaokes of "Wigton shire. Hicks calls it greenish sand- 

 stone, Geikie green shale ; it is of an intermediate constitution, but I adopt the term 



