300 H. HICKS ON THE PRE-CAMBRIAN ROCKS IN 
Silurian beds altered in contact, the true Dimetian type is seen near 
Ty-mawr and for some distance across (vide Appendix, No. V1.). 
Eastward of this, and as we approach the so-called altered Cambrians, 
rocks of a more felsitic character come in abruptly, and it is pro- 
bable that these are of Arvonian age. Both groups are largely pe- 
netrated by intrusive gabbros, but not to the extent marked in the 
Survey maps, especially not so much so to the north of the main 
road across. The Dimetian axis here seems to run in a direction 
almost due N. and 8. Its eastern edge is a line of fault or faults, 
and along its western edge newer Pre-Cambrian rocks rest upon it. 
This condition of faulting along one side of these old rocks, with 
the entire loss of the succeeding groups, is such a frequent occur- 
rence that it seems necessary to refer specially to the physical con- 
ditions or probable causes which produced this effect. One thing 
which has become particularly evident during our researches is the 
fact that all bits of Pre-Cambrian rocks which have been included 
in succeeding sediments must have been not only in an indurated 
condition when broken off from the parent rocks, but, moreover, that 
they had even then undergone metamorphism, and the more slaty 
ones a species of cleavage. The lowest Cambrian rocks found 
are made up of bits and pebbles of these rocks; and so like are they 
frequently to the solid rocks below, that there can be no doubt 
that they were the beach-pebbles when those old rocks formed 
coast-lines. Now these old recks must have undergone gradual 
depression to receive the subsequent sediments ; and as this depres- 
sion could not take place in rigid or metamorphosed rocks without 
producing fractures, we have at once one cause for some of the 
faults, and reasons for coast-lines continuing for a considerable 
period in some cases, whilst the surrounding areas were becoming 
depressed to a great depth. The greatest faults, however, and 
those which we have most frequently to deal with, are those which 
occurred after the succeeding Cambrian and Silurian sediments 
were deposited. During the great contractions of the crust in 
Paleozoic time, especially towards its close, the rigid Pre-Cambrian 
erust could not fold, enormous fractures would take place instead, 
and the overlying rocks would be thrown down. In some cases, 
as found here and at other places, the Pre-Cambrian would be 
brought to the surface along one edge of the fracture, and its other 
edge would be depressed to a great depth. The fault in Ramsey 
Island, at St. David’s, has a downthrow of over 16,000 feet; and 
I think the one here can hardly be less—that is, if the usual sedi- 
ments found in other areas in Caernarvonshire were ever deposited 
here, and there seems no reason to suppose that they were not. 
According to this view beds belonging to many different horizons 
in geological succession would now appear on the surface faulted 
against the Pre-Cambrian rocks; and this it is that occurs 
wherever they can be examined. I have found beds occupying 
every position from the Lowest Cambrian to the Bala beds in direct 
contact with the Pre-Cambrian rocks as the result of faults. The 
constant recognition of these facts has been of great value in at- 
