Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/221

 next ; for Protospongia fenestrata occurs in the Longmynd group, in the Menevian group, and again in the Upper Lingula flags, to the base of the Tremadoc rocks, where it was found by me a few years since when examining these rocks in North Wales. These therefore continued to live on during the deposition of from 8000 to 10,000 feet of strata. None of the other Menevian species have any considerable range ; and but very few of the genera pass beyond the Menevian boundary. Out of ten genera of Trilobites, one only, the little Agnostus, passes upwards*, and not one species out of the whole number, in all twenty-nine. Indeed the range of all these species is very limited ; and each one seems to mark a special zone, where it flourished for a time and then disappeared, perhaps to be followed by others of the same type, but never to reappear. The Crustacea are therefore, in these earliest rocks, the surest indices of the age of the strata, and the best guides in defining the several zones ; for the more perfect the forms and the higher the development, the less likely are they to have a great range.

It seems difficult to conceive that distinct and separate creations should so frequently take place ; and yet how strange that, up to the present time, no satisfactory cause has been assigned for this constant dying out of old forms and replacement by new ones! It seems possible to conceive that physical conditions should occasionally produce sudden destruction or disappearance, for a time, of particular forms, and a recurrence with a reappearance of the same conditions —as for instance when a sudden elevation of a portion of the sea-bottom took place, forming a beach or shallow water, and then became again depressed. But when we have an even deposit, with some of the forms continuing while others are dying out and new ones coming into the field, we must look for other than physical conditions, and for some cause hitherto unexplained. Evidences of frequent oscillations of the sea-bottom during the deposition of the Cambrian rocks are of common occurrence in North and South Wales. The results obtained by the examination of the successive strata in the neighbourhood of St. David's may be briefly summed up thus† : —

1. A pre-Cambrian island, composed of quartziferous beds interstratified with dark green sandstones, and with a strike from N.W. to S.E., and hence discordant with that of the overlying Cambrian strata. This is possibly one only of a group which existed at this period in this neighbourhood ; for another occurs, under almost the same conditions, about eight miles from St. David's, and surrounded by beds much like those at St. David's.

2. Fine-grained quartziferous hornstones of a bright green colour, with a strike from N.E. to S.W., and resting on the sides of, but not overlying, the pre-Cambrian island. During the deposition of these

that the so-called Conocoryphe of the upper rocks is generically distinct from the Menevian ones.
 * Conocoryphe also is mentioned ; but I agree with Mr. Belt in thinking

† I have already given the thickness of each division at p. 391, Quart. Journ, Geol. Soc. vol. xxvii.

VOL. xxviii. — Part I.  O