Most of the Cambrian of Arvon has the “Caledonian” strike which is dominant in Snowdonia. But there is a tract at Tregarth, about a square mile in extent, where the beds trend in any direction: that is, have no strike at all. Nothing in the beds themselves is any explanation whatever: there seems no reason for this anomaly. We have to seek an explanation in the underlying floor, and there, I think, we win some light. That floor has more than one component: Mona Complex and Arvonian. In the chapter of the Anglesey Memoir on the Age of the Mona Complex, I have thrown out a suggestion (then in explanation of nothing but the pebbles), that hills of Gwna Quartzite stood up through the Arvonian lavas. Now, in the great conglomerate of the crest and dip-slope of Y Pare, pebbles of quartzite are so numerous as to outnumber those of the rhyolite. It seems legitimate to infer that, underneath this Tregarth tract, these buried knobs of quartzite were exceptionally numerous and exceptionally high, and made the sub-Cambrian floor so uneven that the beds which rested on it could not acquire the usual strike, but were driven hither and thither, in all sorts of irregular ways. The same irregularity, though to a somewhat less degree, affects the Arvonian tuffs themselves.