58 



NATURE 



[Nov. 27, 1873 



latter only one specimen, in the form of a tail, has yet 

 been obtained from the Upper Llandeilo strata of the 

 South of Scotland ; and this specimen is too imperfect to 

 admit of its being specifically determined. With refer- 

 ence to molluscs, these are nearly equally rare in the 

 Southern Uplands. Only two Brachiopods have hitherto 

 been recognised, while many forms appertaining to several 

 genera have been obtained from the Welsh Upper Llan- 

 deilo strata. Notwithstanding the paucity of varied 

 forms of organic remains in the Upper Llandeilo rocks 

 of the Southern Uplands, their rich graptolitic fauna is 

 at once indicative of their age, and the absence of other 

 forms is most probably referable to want of calcareous 

 strata in connection with these deposits. 



The labours of the officers of the Geological Survey 

 among the highly contorted and crumpled rocks of the 

 Southern Uplands have afforded further information, were 

 such required, of the causes from whence cleavage results. 

 In a country so subject to flexures and contortions, where 

 anticlinal axes and synclinal folds have been inverted, we 

 should naturally look for abundant evidence of the super- 

 induced structures from which true slates have derived 

 their origin. The great mass of the Upper Llandeilo 

 rocks of the South of Scotland rarely furnishes anything 

 in the form of slates proper ; and when we consider the 

 nature of these rocks, which consist for the most part of 

 greywacke sandstones and grits, we cannot fail to dis- 

 cover that the cause of the general absence of cleavage 

 from these rocks has arisen from their petrological 

 nature. The officers of the Survey have, however, in several 

 instances, pointed out the recurrence of cleavage among 

 the finer shales ; and this occurrence usually accompanies 

 violent contortions of the strata. 



Although rocks of an Upper Llandeilo age enter so 

 largely into the composition of the Southern Uplands, 

 they are not the exclusive representatives of the Lower 

 Silurian rocks in this area ; above the Upper Llandeilo 

 strata rocks referable to the Bala or Caradoc age occur. 

 These Caradoc rocks, which occupy a very small area 

 when contrasted with the Upper Llandeilo strata, are 

 marked in the Southern Uplands by a feature which is 

 unknown to their occurrence elsewhere. They are jiiicon- 

 forinablc to the underlying Upper Llandeilo beds, a cir- 

 cumstance which Prof. Geikie well describes as " a new 

 feature in the geology of Britain." The Caradoc rocks 

 have not been recognised in Wigtonshire. They are 

 described in connection with Sheet 15. They occur in a 

 trough extending from Wedder Dod N.E. at least as far 

 as the hills on the right bank of the Clyde, below Abing- 

 ton in Lanarkshire. 



Here they are seen as greywackes, "passing on the one 

 hand into a crumbling sandstone, and on the other into 

 pebbly grits, with shale partings and with beds of con- 

 glomerate found chiefly at their base." In one spot a little 

 concretionary limestone is seen, " the only example of 

 limestone met with in the Lower Silurian rocks in .Sheet 

 15." This limestone has afforded no fossils, but the con- 

 glomerates and the pebbly and gritty beds higher up in 

 the series are abundantly fossiliferous. Denudation has 

 probably removed some higher beds from this group. Its 

 total thickness amounts to about 1,700 feet. 



From the Caradoc rocks of the Lead Hills the geolo- 

 gical surveyors have obtained a good series of fossils. 



We miss from their list the whole of the graptolites so 

 abundant in and so characteristic of the Upper Llandeilo 

 strata. In their place we have corals, trilobites, many 

 forms of brachiopods, two lamellibranchiates, several gas- 

 teropods, and an arthorceras. Most of the species are 

 characteristic Caradoc forms ; but they have associated 

 with them some which occur also in the Llandovery series. 

 The Southern Uplands of Scotland have other mem- 

 bers of the great Silurian series besides those which have 

 been referred to. These occur along a portion of the 

 south-east flanks of the range, and consist of rocks having 

 a general resemblance to the greywacke strata which form 

 so large a part of the Upper Llandeilo rocks in the South 

 of Scotland. The newer Silurian strata occurring on the 

 south-east margin have, however, a very distinct series of 

 fossils ; and associated with their shales are found calca- 

 reous concretions frequently affording organic remains ; 

 the greywackes flaggy beds also in this higher group 

 often contain fossils, especially graptolites. These grap- 

 tolites belong to species occupying a much higher horizon 

 than the forms which make their appearance in the Upper 

 Llandeilo rocks ; and the organic remains derived from 

 the calcareous nodules also indicate strata.higher in posi- 

 tion than the Caradoc series. The rocks of an Upper 

 Silurian age are well developed on the shores of the 

 Stewarty of Kirkcudbright, especially on the eastern side 

 of the mouth of the Dee. They occur also in Dumfries- 

 shire, being seen near the southern margin of the Silu- 

 rians at Dalton Mill, in the parish of Dalton, where the 

 flaggy strata yield the same forms of graptolites which 

 occur near the mouth of the Dee ; and they have been 

 extensively recognised in Roxburghshire. 



As contrasted with the nearest area where Silurian 

 rocks occur in England, the strata and the organic re- 

 mains of the Southern Uplands of Scotland show great 

 dissimilarity. 



The distance of the nearest portion of the area where 

 Silurian rocks are seen in England from the south-east 

 side of the South of Scotland strata of the same series 

 does not exceed 30 miles ; for the northern flank of the 

 Caldbeck range in Cumberland is not greater than this, 

 in distance from the axis of the Lower Sdurian rocks in 

 Dumfriesshire where the Ardwell group occurs. 



The Lake district of the north of England, occupied 

 principally by Silurian rocks, exhibits strata of a lower 

 position than any of the Silurian deposits of the Southern 

 Uplands. These lower rocks of the Lake district are the 

 Skiddaw slates of Prof. .Sedgwick, which in many locali- 

 ties contain graptolites. 



The facies of this graptolitic fauna is, however, widely 

 different from that of the graptolitic fauna of the Upper 

 Llandeilo rocks of the south of Scotland. In the Lake 

 district there are no strata which can be paralleled with 

 the Upper Llandeilo rocks. Above the Skiddaw slates of 

 the north-west of England there occur great accumula- 

 tions of igneous rocks in the form of traps, ashes, trap- 

 tuffs and similar volcanic products. And it is only when 

 the highest of these rocks is reached, which appear to 

 have resulted from sub-aerial volcanic action, that strata 

 occur in which organic remains are met with. 



These strata, the Coniston limestones and their asso- 

 ciated shales, are prolific in fossils of a nature indicative 

 of the Caradoc age. 



