158 James Neilson — Old Red and Carboniferous of Arran. 



No time need be lost in discussing the beds described by Sedgwick 

 and Murchison as Old Red Sandstone — that point is admitted on 

 all hands ; the question is, whether the beds considered by them 

 and subsequent writers to be Carboniferous, are really so, or Old 

 Red Sandstone as Sir A. Geikie contends. For this purpose we 

 come to the junctions of these formations, and find that both 

 on the north and south the Old Red Sandstone conglomerates are 

 succeeded by calcareous conglomerates and limestones (cornstones), 

 alternating with sandstones. These strata overlie the Old Red 

 Sandstone conglomerates conformably, and if Arran alone had 

 been in question it is doubtful whether the line would have been 

 drawn here. Unfortunately the strata at the south end are cut off 

 about 40 yards from this junction by a dike whose cross section 

 extends over 320 yards along the shore, and as it seems to us 

 that this dike marks a fault cutting off a considerable thickness 

 of strata we are compelled to fill in the gap by reference to the 

 corresponding northern section, where we find this same cornstone 

 series overlain by beds of sandstone, black shale, and volcanic ash. 

 This shale contains charred plant-remains belonging to Calamites, 

 Lepidodendron, Lepidostrobns, also a species of Spirorbis. besides, what 

 is even of more interest, it contains the remains of fishes. These 

 have been submitted to Dr. Traquair, whose report is to the effect that, 

 while the specimens were too fragmentary to identify specifically, 

 still one basal part of a tooth probably belonged to the genus Rhi- 

 zodus ; another tooth resembled Strepsodus striolatus, Traq. There 

 are also an imperfect Rhizodont scale, and some scales probably 

 belonging to Rhadiuichthys, Traq. He concludes his report as 

 follows: "Although the specimens are, unfortunately, not such as 

 to permit me to give any list of species, it is to be noted that they 

 exhibit an unmistakably Carboniferous facies." 



Overlying this shale is a bed of volcanic ash, also containing 

 jish-rem&ins, consisting of fragments of bones and scales — some 

 pretty large, but I was not fortunate in finding anything in a 

 condition to admit of being named. The information with regard 

 to this latter bed was obtained from Mr. James Thomson, F.G.S., 

 whom I believe to be the discoverer, and it was in searching 

 for it that I stumbled across the black shale above mentioned. 

 It speaks volumes for the accuracy of the old writers that here, 

 within a few yards of the line they drew as separating the 

 Old Red and Carboniferous formations, should be found a bed 

 charged with characteristic Carboniferous fossils. This is also 

 the lowest horizon, so far as I am aware, where Carboniferous 

 fossils have been found, and, as will afterwards be shown, it 

 occupies a position several hundred fathoms below the marine 

 limestones which were formerly grouped with the Old Red Sand- 

 stone formation. 



Proceeding onwards, it should be remembered that owing to the 

 anticline the rocks dip away from the observer in the direction in 

 which he is travelling, so that he sees first only the upturned edge 

 of the rock. It will thus be evident that each bed met with is higher 



