Moimtain Limestone in North Wales. 527 



near these places, for neither Mold nor Holywell are actually upon 

 the Mountain Limestone. Llangollen was included because of the 

 grand section presented by the Eglwyseg rocks, about a mile from 

 the town. 



The estimated thickness of the Limestone was 1,200 feet. The 

 Millstone Grit of the Geological Survey succeeds the Limestone, and 

 is about 800 feet thick, coal being worked above it at Tyfynuchaf 



One mile and a half to the west of the town of Mold there is a 

 fine section of the Mountain Limestone. The vertical section of the 

 strata at Newmarket is compiled from three horizontal sections 

 which each show the thickness of a subdivision. The upper, middle^ 

 and lower limestones are on the surface disconnected by faults, but 

 the relative geological position of each is obvious. The lower grey 

 and black limestones rest on Silurian strata, and are 750 feet thick 

 at Moel Hiraddug.. The white limestone rijdge, 350 feet thick, is 

 very different from any of the Hiraddug strata, as it also is from the 

 black and grey limestone and shales, 300 feet thick, which crop out 

 from under the overlying shales and sandstones of the Millstone 

 Grit. Consequently the white limestone of Ax,ton occupies an 

 intermediate position, which was confirmed by the fossils it con- 

 tained.. Appended to Mr. Morton's paper was a list of ninety-four 

 fossils found in the district which he had described, and which showed 

 the range of the specifis through the three (upper, middle, and lower) 

 divisions of the Carboniferous Limestone. When the number of 

 species is considered, the upper and middle divisions are seen each 

 to have many peculiar fossils, and an undue distinctive character 

 might be supposed to separate them. Bu^t when the relative abun- 

 dance of the species is considered, this distinction is not marked, for 

 the common forms have a considerable range,, while the rare ones. 

 are more or less restricted. 



An analysis of the list showed some interesting results. Ex- 

 cluding a few fish teeth and scales, and plants, aU of which occur in 

 the upper limestone, it appeared that the lower limestone does not 

 contain any species peculiar to it, for some of them extend upwards 

 into the white or middle limestone, whilst others pass up to the 

 highest subdivision. The earlier species, principally found at the 

 base of Moel Hiraddug, are Spirifera lineata and Syringopora retieu- 

 lata, and they continue upwards through the whole of the Mountain 

 limestone. These two species, with a Lepidodendron and another 

 plant, seem to have been the first colonists that settled down in the 

 Carboniferous sea of North Wales. The middle or white limestone 

 presents twenty-eight species which are peculiar to it (of course 

 common species elsewhere), but of these no less than twenty-three 

 have only been found in the limestone ridge at Axton, Newmarket. 

 This assemblage of species in such a limited area was extraordinary. 

 In the upper limestone, twenty-three species have been found to be 

 peculiar to it, but they are all of rare occurrence, only single speci- 

 mens having been found of about half of them. 



