Page:The Geologist, volume 5.djvu/44

30 showing the very slender organic connection between the deposits under notice and those of the Silurian age. A glance at the Table shows that, of the two, Petherwin is the nearest to the Lower Devonian horizon, and the most remote from the Carboniferous; true, the majority in each case is but small—208 to 171, and 211 to 181—but it must be remembered that great ones were not expected; and that, feeble as they are individually, there is strength in the fact that their testimonies agree; if they mean anything, it is that the Barnstaple beds are somewhat more modern than those of Petherwin; a conclusion to which more than one eminent geologist has been led by other, and, perhaps, more reliable evidence.

The fossils of the two areas belong to forty-six genera, of which thirty-three are represented by the Petherwin, and thirty-four by the Barnstaple series, twenty-one are common to both; hence twelve are peculiar to Petherwin, and thirteen to Barnstaple. The South Devon and contemporary beds contain sixty-four genera, of which thirteen only occur in the deposits now under notice.

Taken as a whole, the forty-six genera above mentioned have a Carboniferous, rather than a Silurian, or even a Lower Devonian facies. They may be divided into groups, namely, 1st, those characterized by a considerable maximum specific variety or development in some one period before or after Petherwin and Barnstaple times, that is, during the Silurian or Lower Devonian eras on the one side, or the Carboniferous on the other; 2nd, those that are not thus distinguished. For example, the rich genus Orthoceras had, in Britain, an almost equal number of species in Carboniferous and Upper Silurian times, when it was richest; hence it had no one period of maximum specific variety, and consequently belongs to the second of the groups just defined; as, of course, do also all other genera similarly characterized, as well as those, such as Hallia, which seems never to have had more than a very few species at any one time.

The first of these groups—which alone we have to consider here—contains thirty-one genera, of which six may be said to belong to the Past, and twenty-five to the Future, the age of Petherwin and Barnstaple being the chronological stand-point.

The first, or "Past" division, does not contain a number sufficiently great to be of service in this inquiry. The last, or "Future," consists of two series, namely, 1st, those genera which are equally represented in the two sets of beds; and 2ndly, those that are not; evidently the last series alone can supply information on the question under consideration. It is made up of the fifteen genera named in the following table, in which the columns headed P., B., C, exhibit the number of species, belonging to each genus, which occur in the Petherwin, Barnstaple, and British Carboniferous beds respectively.

Prom the table we learn that nine of these genera are found in Barnstaple only, or are more largely represented there than in Petherwin; and that nineteen species represent the ten genera found in the former area, and no more than ten the six genera of the latter. Hence, the genera tell us what the species had told us be-