Downloaded from http://trngl.lyellcollection.org/ at University of Auckland on March 24, 2015 Part iii.] MACNAIR—HURLET SBQUBNCB IN N. LANARK. 387 No. XXIII.—THE HURLET SEQUENCE IN NORTH LANARKSHIRE. By PETER MACNAIR, F.R.S.E., F.G.S. [Read 13th May, 1915; issued separately 10th February, 1916.] I. INTRODUCTION. THE Lower Limestone Series in the area about to be described lie to the north of a line drawn from Drumclog, east to Lesma- hagow, and thence north-east by Kilncadzow to the Cobbinshaw Reservoir on the confines of Lanarkshire and Midlothian. They form a lip or edge of the basin which has the great Coal Measure Series of Lanarkshire in the centre, and they are bounded on the west and south by the volcanic plateau of Eaglesham and Strathavon, which separates them from the Ayrshire basin, and on the south and east by rocks of Lower Old Red Sandstone and Calciferous Sandstone age. Though now separated from the Lower Carboniferous Limestone area of Renfrewshire and Ayrshire by the volcanic plateau of lavas, at one time the two areas must have been more or less con­ tinuous, as can be shown by the occurrence of such outliers of the limestone as we find at Ryeland and Drumclog resting directly upon the lavas. The former continuity of the two areas is also clearly brought out by the identity of the lime­ stone series on each side of the volcanic ridge, as we hope to be able to show that such well-marked horizons as the Dockra or Blackbyre Limestone, the Blackhall Limestone, the Calder- wood Cement or top Hosie Limestone, the Slingstone Limestone or top marine band, all with characteristic faunal associations, can be traced over many hundreds of square miles from the neighbourhood of Dairy on the west to the confines of Lanark­ shire and Midlothian on the east. An examination of a geological map will show that the district under consideration includes many of the most notable fossil localities in the west of Scotland. In the East Kilbride district we have such localities as Thorntonhall, Braehead, Craighall, Crosshouse, and South Shields all in the main Lime­ stone then we have the exceedingly numerous and richly fossili- Downloaded from http://trngl.lyellcollection.org/ at University of Auckland on March 24, 2015 388 TRANS. GEOLOGICAL SOCIETY OP GLASGOW. [Vol. XV. fercus horizons of the Calderwood Cement series exposed in the neighbourhood of East Kilbride, Calderwood Glen, and the Gill Burn at Jacktown. In the Strathavon district the locali­ ties at Gallow Hill, in the Netherfield Limestone, and at Waulkmill, in the Blackhall or Foul Hosie Limestone, have also been long known. In the Lesmahagow district the quarries at Boghead and the sections in the Birkwood and Teiglam Burns have yielded extensive suites of fossils to the west of Scotland collectors. When we come to the Carluke district the fossil localities are even richer and more numerous than in those districts just referred to, large collections of fossils having been made from the various horizons from the Productus giganteus Limestone up to the Slingstone Limestone by Dr. Rankin, Dr. Hunter, of Carluke(8),* and others. In all these districts the limestone was formerly extensively quarried, but many of these quarries are now filled up or are rendered in other ways inaccessible. Still the numerous artificial and natural sections still available are of the greatest value in working out the sequence. All round the margin of the basin there is a number of quarries which help to elucidate the structure of the ground. There are also numerous more or less continuous stream sections of the greatest value, such as those seen in the Kittooh Water, Calderwood Glen, the Gill Burn, near Jackton; the Avon and its tributaries between Strathavon and Stone- house, the Birkwood and Teiglam Burns at Lesmahagow, and Fiddler's Burn, Nellfield Burn, and Raes Gill, in the Carluke district. II. DESCRIPTION OP SECTIONS. Thorntonhall and Braehead.—These quarries are situated about a mile to the south of Busby, and 5 miles to the south­ east of Hurlet. They lie on the north-west side of the basin, which it is the object of this paper to describe. Along the railway cutting and in the bed of a small stream near Thornton- hall Station the ashy muds, with large pebbles, are visible. The top of the muds can also be seen in the tramway which runs into the mine. Above this comes a grey shale, succeeded by * Numerals in ( ) refer to the list of papers enumerated in the Bibliography at the end. Downloaded from http://trngl.lyellcollection.org/ at University of Auckland on March 24, 2015 Part iii.] MACNAIR—HURLET SEQUENCE IN N. LANARK. 389 a black calcareous shale, which is simply a mass of fossils principally brachiopods of the genera Productus and OrtMs. They are mostly, however, in a very crushed condition. On the top of this shale lies the Main Limestone, 10 feet in thick­ ness, which contains bands of Lithostrotion and the large variety of Productus giganteus. Then follows 2 feet 6 inches of entomostracan limestone and shale, 3 feet of nodular cement with rootlets, a thin parting of entomostracan shale and coal, followed by 1 foot 6 inches encrinital limestone, the whole being capped by shales and thin ironstones, the former con­ taining the rich fauna described by Mr. Neilson(5). About half a mile further to the west at Braehead Quarries a similar but more continuous section may be seen. We have carefully examined the whole length of the somewhat extensive outcrop laid bare in the quarries, and we find that, as a rule, only some 6 feet of the last member of the series can be seen, it being usually topped by a thin layer of boulder clay. It is of great interest, however, to note that Mr. Bennie speaks in his paper(lO) of a supposititious limestone 1 foot in thickness, whose existence is inferred from loose blocks lying on the shale. In a future part of this paper we shall attempt to show that it may be the same bed as is seen in situ at Arrotshole. This section at once reminds us of the Hurlet group of strata that lies between the Blackbyre Limestone and the Hurlet Lime­ stone, and though sections are now somewhat scarce in that district, yet what can be seen clearly goes to prove that we are here dealing with the same two limestones and their associated strata. Undoubtedly the best section of the Hurlet Limestone and the strata immediately underlying it to be seen in the Hurlet district is that exposed in Arkleston cutting, east of Paisley, and a comparison of it with the Thorntonhall and Brae­ head sections shows the close resemblance of the strata at the two localities(16). We have shown that what in all probability is the Blackbyre Limestone cropped out to the west of the bridge over the railway at Arkleston. This we compare on palseontological and stratigraphioal grounds with the Main Limestone of Thorntonhall. At Arkleston the Blackbyre Lime­ stone is succeeded by a band of nodular limestone associated with fireclay shale and thin bands of dark impure entomostracan Downloaded from http://trngl.lyellcollection.org/ at University of Auckland on March 24, 2015 390 TRANS.—GEOLOGICAL SOCIETY OP GLASGOW. [Vol. XV. limestone, which have all the characteristics of the sediments lying above the Main Limestone at Thorntonhall. The succeed­ ing limestone at both localities we take to be the Hurlet Limestone. Phillipshill.—The next section we have to consider lies about 1 mile to the east of the Thorntonhall Quarries and is here mentioned principally for the purpose of proving the continuity of the outcrop toward the important section exposed still further to the east in the Kittooh Water. The old workings have now been considerably grassed over, but taken along with an exposure seen in a small burn one is left in no dubiety as to the identity of the strata with those seen at Thorntonhall. The pebbly, ashy muds are seen in the burn to pass up into the highly fossiliferous shale at the base of the limestone which has been worked out. The overlying sediments and nodular limestone are succeeded by the top limestone of Thorntonhall. Kittoch Water.—We now come to the important section ex­ posed on the Kittoch Water from Arrotshole to Little Dun- canrig, in which all the principal members of the Lower Limestone Series can be traced(12). Beginning at Arrotshole, we find the Main Limestone of Thorntonhall and Phillipshill resting directly upon the ashy muds with large pebbles, the latter well exposed on the right bank of the stream a little below the waterfall. Further up stream we find the shales and impure limestone with large numbers of entomostraca, the characteristic nodular limestone with rootlets, and No. 2 limestone of the previous sections, which here makes a second waterfall. Above this limestone comes the black shales, yielding an identical fauna with that described from Thorntonhall by Mr. Neilson. The most abundant shell is Spirifera urei, which here occurs in great numbers, but which is a comparatively rare shell on other horizons. The accompanying fauna is so strikingly like that seen at Thorntonhall, that taken along with the strati- graphical evidence the identity of the respective beds is unquestionable. These shales can be traced more or less continuously up stream to a third waterfall, below which they appear to pass. This waterfall is formed by a third crop of limestone, and a careful examination of the strata seen in the stream both above and below it convinces us that we are here Downloaded from http://trngl.lyellcollection.org/ at University of Auckland on March 24, 2015 Part iii.] MACNAIR—HURLET "SEQUENCE IN N.