Horizon of Dr. Scouler's Dithyrocaris. 273 



Gallowhill Limestone with that which forms the matrix in which 

 Scouler's fossils are preserved. The most abundant fossil in the 

 Gallowhill Limestone is Lingula mytiloides, many of the slabs being 

 simply crowded with them. The full significance of the faunal 

 association found in the Gallowhill Limestone will, however, be 

 discussed in more detail presently. In his paper Scouler refers to 

 the absence of Productidse, corals or stems of crinoid animals, but 

 Prodnctus longispinus and Rhynclxonella pleurodon are fairly abundant 

 on some slabs. Slabs of the limestone rich in ^rinoidal remains 

 have also been observed. But whether these represent the same bed 

 as that which carries the Phyllopod fauna or one on a slightly 

 different horizon we have not yet been able to determine definitely, 

 but we are inclined to favour the latter view. 



Arkleston Cutting and Former Exposures at Gallowhill. 



In my paper on the Hurlet sequence in North Ayrshire I have 

 given an account of the strata exposed in the Arkleston Cutting, 

 and it is there shown that the top limestone is the Hurlet Limestone, 

 here 3 to 4 feet thick and dipping towards the east. Below it comes 

 the Hurlet Alum Shale, only some 6 inches thick, followed by. the 

 Hurlet Coal, originally from 5 to 6 feet thick. The coal has been 

 split up the centre by a sill of dolerite, some 80 feet in thickness, 

 lenticles of the coal occurring within the sill. Below the sill comes 

 some 18 feet of shale and fireclay, in which there is a limestone in 

 three seams full of entomostraca and fish remains, and which is 

 clearly the equivalent of the Baldernock Limestone of the Campsie 

 district. The whole group of sedimentary strata is much pyritized 

 and altered by contact with the intrusive sill of dolerite. A search 

 for the alum shale fauna in the fragments that are seen to lie above 

 the sill has not as yet been successful in yielding any examples of 

 that characteristic fauna, as the shale is very completely baked. 

 But the discovery of the fauna at this point is not yet regarded as 

 entirely hopeless. I have also shown in the paper mentioned that 

 a highly fossiliferous limestone was at one time exposed between 

 the bridge and the signal box, and that, dipping towards the east, 

 it passed beneath the strata in the cutting just described. A con- 

 sideration of all the available palseontological and stratigraphical 

 evidence goes to show that this limestone must have been the 

 equivalent of the Blackbyre Limestone. 



If we turn to the 6-inch-to-the-mile Geological Survey map, of 

 which Fig. 2 is in part a reproduction, it will be noticed that the 

 limestone, there shown as trending northwards towards the Arkleston 

 Print "Works, and then bifurcating into two outcrops towards the 

 north-west, clearly lies below the sill now seen in the cutting. When 

 the Geological Survey map was made in the year 1875 the railway 

 ran through a tunnel at this point, and the Hurlet Limestone lying 

 above the sill does not appear to have been exposed. If it was it is 

 not indicated on the map. The limestone represented on the map is 

 drawn as two narrow outcrops at the Paisley end of the tunnel. 

 Unfortunately no exposure can now be observed between the bridge 

 over the railway and the signal box. But that a limestone was at 



DECADE VI. — VOL. IV. — NO. VI. 18 



