Dr. H. Hicks — Effect produced by Earth-movements. 559 



fully understood. The difficulties experienced by geologists who 

 visit these areas for the first time are mainly due to their being 

 unable or unwilling to recognize the extraordinary effects produced 

 by these Earth- movements, and especially to the complications 

 resulting from faults and thrusts. In these areas it commonly 

 happens that portions of the pre-Cambrian rocks are forced in 

 among the Lower Palaeozoic rocks so as to appear either to be 

 parts of the series or to be intruded into it. In other places it 

 occasionally happens that not only are lower beds of the same series 

 made to appear to overlie much newer beds, but the pre-Cambrian 

 rocks are, in some cases, thrust over those of Palaeozoic age. 



The rocks in the neighbourhood of St. David's, Pembrokeshire, 

 have suffered in a remarkable degree from the effects of Earth- 

 movements, and some geologists who have visited that area have 

 been led to make not only serious errors, but some very extraordinary 

 statements, from being unable to recognize this fact. 



A section across the St. David's promontory shows that after the 

 Cambrian and Ordovician rocks had been gradually deposited over 

 an irregular surface of pre-Cambrian rocks, great Earth-movements 

 took place which caused the Cambrian and Ordovician rocks to be 

 bent into great folds. These folds were broken during these 

 movements by numerous faults, some of great magnitude. Sub- 

 sequent denudation has revealed the presence, within these folds, 

 of the pre-Cambrian rocks on which the newer rocks were deposited, 

 and has enabled us to recognize the enormous dislocations of the 

 strata which have taken place, and the changes induced in the 

 rocks during the movements. Although there are evidences of 

 several folds of Lower Palaeozoic rocks, containing cores of pre- 

 Cambrian rocks, in North Pembrokeshire, it will be sufficient to 

 refer to the one which occurs in the promontory of St. David's, 

 taking a line across it from St. David's Head on the N.W. to St. 

 Bride's Bay to the S.E. of the city of St. David's. At, and near, 

 St. David's Head, great masses of intrusive basic rocks traverse the 

 Arenig rocks, in a direction nearly parallel with the lines of 

 bedding, being probably portions of sheets in the form of Laccolites. 

 These, along with the slates, have suffered from the pressure, proving 

 that the folding took place after they were intruded into the slates. 

 The Arenig rocks are separated from the Tremadoc rocks along this 

 line by a thrust-fault which has diminished the thickness of the 

 strata by about 1500 feet. Other faults in the arch-limb of this fold 

 have repeatedly caused newer rocks to be thrust forward to hide the 

 underlying series, and near the axis, beds at least 5000 feet apart in 

 the succession are by that means brought into contact. On the 

 S.E. side of the axis the Cambrian beds have been inverted and 

 faulted so as to cause the older beds to overlie the newer, evidently 

 by movements from N.W. to S.E. 



In the pre-Cambrian core itself the Pebidian rocks are not only 

 sheared to an enormous extent, but are also made, on the S. side, 

 by reversed faults to appear to lie under parts of the granitoid rocks 

 (Dimetiau). This result causes the Dimetian to look in places as 



