Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/533

 Sound, these grey fossiliferous grits are not seen : a fault having an E.N.E. course has cut through the upper portion of the purple- coloured rocks which support the grey strata, A few beds of these grey grits, however, are seen on the N.N".W. side of this fault, near Ogaf Golhfa, in Whitesand Bay, reposing upon the higher members of the purple flags and sandstones. The order of the rocks from the quartziferous breccias upwards, when not disturbed by faults, is as follows : —

Lower Cambrian.

feet.

1. Greenish hornstones on the S.E., and earthy greenstones on the N.W., forming the outermost portions of the so-called Syenitic and Greenstone ridges.

2. Conglomerates composed chiefly of well-rounded masses of quartz imbedded in a purple matrix 60

3. Greenish flaggy sandstones 460

4. Red flaggy or shaly beds, affording the earliest traces of organic remains in the St. David's Promontory — namely, Lingulella ferruginea, Leperditia cambrensis, a larger Lingulella, and a Discina 50

5. Purple (sometimes greenish) sandstones 1000

6. Yellowish-grey sandstones, shales, and flags containing the genera Plutonia, Conocoryphe, Microdiscus, Agnostus, Theca, Protospongia, and Paradoxides 150

7. Grey, purple, and red flaggy sandstones, containing most of the above- mentioned genera 1500

8. Grey flaggy beds, containing Paradoxides aurora 150

9. The true beds of the "Menevian Group," richly fossiliferous, and the probable equivalents of the lowest portions of the Primordial Zone of M. Barrande 550

The discovery of a fauna specially rich in Trilobites among the purple and green rocks and their associated strata of the St. David's promontory affords very important information concerning the earlier forms of life which occur in the old sedimentary deposits of the British Isles. Until the discovery of this fauna, these rocks and their equivalents in North Wales have been looked upon as all but barren in fossils. We have now scattered through about 3000 feet of purple and green strata a well-marked series of fossils such as have nowhere else been obtained in the British Isles.

In the Longmynds of Shropshire, consisting of purple and green rocks, which probably represent the rocks having the same colour in the St. David's district, the only evidence of the existence of life during the period of their deposition is in the form of worm- burrows — and in the somewhat indistinct impressions which Mr. Salter regarded as trilobitic, and to which he has given the name Paloeopyge Ramsayi *.

If we assume the purple and green shales and sandstones with their associated quartz rocks of Bray Head, and the drab shales of Carrick McReily, co. Wicklow, to represent the old rocks of the St. David's promontory, they afford only very meagre evidence of the occurrence of life during the period of their deposition, in the form of worm-burrows and tracks and in the very indeterminate fossils which have been referred to the genus Oldhamia.


 * Quart. Journ. Geol. Soc. vol. xii. p. 249.