344 partly into isolated inland water—basins, the sea outside still contained an Upper Silurian fauna, which was ready on any fcavourable opportunity to re-enter the tracts from which it had been excluded. The interval of its reappearance seems to have been very brief, however, for the band of shale con- taining these Upper Silurian marine organisms is only a few inches thick, and the fossils have not been detected on any other horizon. With these exceptions, the fauna of the form-.ition consists entirely of fishes and crustaceans. -.'ine or l‘_1JX‘e species of crustaceans have been obtained, chiefly eurypterids, but including one or two phyllopods. The large pterygotus (P. .fn._qlicus) is especially charac- teristic, and must have attained a great size, for some of the individuals indicate a length of 6 feet with a breadth of l§ feet. There occur also a smaller species (P. minor), two l2'zn-_z/pteri, three species of iSt_1/lonurus, and abundant clusters of crustacean egg-packets termed I’ru-I-ia clecipiens. Seventeen species of fishes have been obtained, chiefly from the Arbroath flags. They belong to the snborders Arzmtlzoclidte and Ostmcostei. One of the most abundant forms is the little .fc(mt/wdes J1 itr:/tell-i. Another common fish is Diplucantlms gracilis. There occur also ClimaI2'us scutiger, C. reticulatus, and C. 'unc2'natu.s, I’are.rus 2'nczu'z'u.s, Eu!/tacant/ms (four species), Ceplzalaspis L3/ellii, and Ptar- aspis fllitchelli. Some of the sandstones and shales are crowded with indistinctly preserved vegetation, Occasionally in suflicient quantity to form thin laminw of coal. In For- farsliire the surfaces of the shaly flagstones are now and then covered with linear grass-like plants like the sedgy vegetation of a lake or marsh. In Perthshire certain layers occur chiefly made up of compressed stems of Psilop/L3/ton. The adjoining land-was doubtless clothed with a flora in large measure lycopodiaceous. The Old Red Sandstone of the northern basin is typically developed in Caithness, where it consists chiefly of the well- known dark-grey bituminous and calcareous flagstones of commerce. It rests unconformably upon the metamorphosed Lower Silurian schists, and must have been deposited on the very uneven bottom of a sinking basin, seeing that mcasionally even some of the higher platforms are found resting against the schists and granites. The lower zones consist of red sandstones and conglomerates which graduate upward into the flagstones. Other red sandstones, however, supervene in the higher parts of the system. The total depth of the series in Caithness has been estimated at upwards of 16,000 feet. Murchison was the first to attempt the correlation of the Caithness flagstones with the Old Red Sandstone of the rest of Britain. Founding upon the absence from these northern rocks of the characteristic cephalaspidean fishes of the ad- mitted Lower Old Red S.mdstone of the south of Scotland and of Wales and Shropshire, upon the presence of numer- ous genera of fis'ies not known to occur in the true Lower Old Refl Sandstone, and upon the discovery of a I’(er_z/- _r/otus in the basement red sandy group of strata, he con- cluded that the massive flagstone series of Caithness could not be classed with the Lower Old Red Sandstone, but must be of younger date. He supposed the red sand- stones, conglomerates, and shales at the base, with their Pte;-yyotus, to represent the true Lower Old Red S.mdstoiie, while the great flagstone series with its distinctive fishes was made into a middle division answering in some of its ich- thyolitic contents to the Middle Devonian rocks of the Continent. This view has been accepted everywhere by geologists. Recently, however, it has been called in ques- tion by Professor Geikie, who gives reasons for maintaining the Caithness flagstones to be Lower Old Red Sandstone, and for denying the existence of any middle division. He shows that the discrepance in organic contents between the Caithness and the Arbroath flags is by no means so strong GEOLOGY [v:. STl1ATlGR_'tPHICAL. as Murchison supposed, but that several species are common to both. In particular, he finds that the characteristically Lower Old Red Sandstone and Upper Silurian crustacean genus I’ter_2/_r/otus occurs, not merely in the basement zone of the C‘-aitlniess flags, but also high up in the series. The genera .~f«-met/zocles and 1)i};Zacunt/ms are abundant both in Caithness and in I"orf-arshire. ]’urc.rus 2'ncurz-us occurs in the northern as well as the southern basin. It is contended that the palzeontological distinctions are not greater than the striking lithological differences between the strata of the two regions would account for, or than the contras-.t between the iclitliyic faunas of contiguous watci'—b-as-iiis at the present time. Somewhere about 60 species of fishes have been obtained from the Old Red Sandstone of the north of Scotland. Among these the genera Accmtliotles, Asterole1,z's, ("/m'ru«-mz- t/ms, C’/m'rolepis, Coccoslcus, 1')z'placcuzt/ms, .l)z'_)»lo1;I¢-2-u.»-, I)iptcrus, Cl‘:/ptolepis, Osteolepis, and I’ter-ic/cl/z_2/s are spe- cially characteristic. Some of the shales are crowded with the little ostracod crustacean I;'st/icria inembranarm. Land- plants abound, especially in the higher groups of the flag- stones, where forms of P.s'ilop/1_a/ton, Lepidoclcndroiz, Stig- 7u:'u'ia, ;S'z'_qillcu'z'a, Calamile, and C3/r-lopleris, as well as other genera, occur. In the Shetland Islands traces of abundant contemporaneous volcanic rocks have been ob- served, which, with the exception of two trifling examples in the region of the Moray Firth, are the only known instances of volcanic action in the Lower Old Red Sandstone of the north of Scotland. In the other two Scottish basins, those of the Cheviot Hills and of Lorne, volcanic action continued long vigorous, and produced thick piles of lava like those of the central basin above referred to. The Upper Old Red Sandstone consists in Scotland of red sandstones, clays or inarls, conglomerates, and b1'ecci-as, the sandstones sometimes becoming yellow or even white. These strata, wherever their stratigraphical relations can be distinctly traced, lie unconformably upon the lower division of the system, and pass up conformably into the L‘-arl_»oni— ferous rocks above. If they are studied from the side of the underlying formation, they seem naturally to form part of the Old Red Sandstone, since they agree with it in general lithological character and also in containing some distinctively Old Red Sandstone genera of fishes, such as I’teric/H.713/s and Iloloptg/c/tiers. But, approached from the upper or Carboniferous direction, they appear rather to form the natura.l sandy base of that system into which they insensibly graduate. On the whole, they are reniarl<abl_v barren of organic remains, though in one loc-ality———])ura ]_)en in Fife—tl1ey have yielded a number of genera and species of fishes, crowded profusely through the pale sand- stone as if the individuals had been suddenly killed and rapidly covered over with sediment. Among the character- istic organisms of the Scottish Upper Old Red Sandstone are I’teric/LI/:3/s major, I[oZo};I_2/r-lu'us vzobilissinzzzs, II. Anclersoni, (z'I_z/}';topom11.s, G'l_1/ptolcumts, and I’/umeroplcuron. An interesting fact deserves mention here as a corollary to what has been stated above regarding the survival for some time of an Upper Silurian fauna outside the area of the British Old Red Sandstone lakes. In the Upper Old Red Sandstone of the basin of the ]"irth of Ulyde, 1’tcric/at/z_;/s major and I[olopt_z/«-Mus occur at the Heads of Ayr, while a band of marine limestone lying in the heart of the red sandstone series in Arran is crowded with ordinary Carboniferous Limestone shells, such as 1’)'u(luCl’u.s' gig/anteus, I’. s(’2nirelir'ulatu.s, 1’. punctatus, C/zonctcs Hard- rensis, Spirz:/‘er lineatus, &'c. None of these fossils has been detected in the great series of red sandstones overlying the limestone. They do not reappear till the limestones at the base of the Carboniferous series; yet the organisms
must liavc been living during all that long interval outside