388 ME. G. H. MOETON^ ON THE CAKBONIFEROUS [Allg. 1 898, 



No doubt the limestone on the south-west has been dolomitized by 

 water containing magnesium carbonate, percolating through the 

 strata from the fault or fracture supposed to run up the valley ; 

 but for some unknown cause it did not alter the limestone on the 

 north-east side. 



A few sections of the Middle White Limestone and Dolomite, 

 from near Pen Morfa Lod^e, were prepared for the microscope by 

 Mr. Beasley, and proved to be either limestone with foraminifera 

 or crystalline dolomite. None of them showed an intermediate 

 condition or a passage from one rock into the other. 



(2) Middle White Limestone. 



The Middle White Limestone rests upon the Lower Brown Lime- 

 stone, and is exposed over most of the Great Orme's Head. The 

 clearest exposure of the lowest beds is between the Old Road and 

 the Plagstaif quarry, north of the Pavilion, where the bed of shale, 

 7 feet thick, is the base and separates the two subdivisions. Above 

 the shale there is a black limestone, about 18 feet in thickness, 

 forming a conspicuous horizon along which the rock has been 

 obtained for building purposes, on account of its occurrence in 

 regular beds from 6 inches to 2 feet thick, with an inch or two of 

 shale between them, allowing the stone to be easily worked. About 

 50 years ago the lofty cliffs were underquarried, until the obvious 

 danger of the town being buried under a great fall of rock prevented 

 further excavation. 



The Middle White Limestone forms the main mass of the head- 

 land, and is about 550 feet in thickness. (In sailing round the 

 coast the overlying subdivision can be scarcely seen, while the 

 underlying subdivision occurs only in the cliffs about Llandudno 

 and the lower part of the Happy Yalley.) It is exposed in the 

 massive beds forming the cliffs above and below the Marine Drive, 

 where it nearly resembles the subdivision in other localities ; it is of 

 a light grey colour, though occasionally of a darker shade. The 

 limestone contains very few partings of shale, some beds of red marl 

 on both sides of the Old Road being exceptional. Near Pen Trwyn, 

 west uf the Drive, there is a conspicuous band of rubble and con- 

 cretionary marl which separates the Middle White Limestone into 

 a lower and an upper portion of nearly equal thickness, for a mile 

 along the coast and inland to Eoft Fach. This divisional line has 

 been introduced in the map (fig. 2, p. 386), though it is difficult to 

 define its exact horizon along the western side of the Head. There 

 are, however, occasional concretionary beds about the north-western 

 exposures of the limestone, and at the Lighthouse, at about the 

 same horizon. The concretionary band at Pen Trwyn consists of 

 red and yellow beds of marl and rubble 6 feet thick, with thin beds 

 of limestone and a higher bed of marl, altogether about 20 feet 

 thick, extending 5 mile along the side of the Drive; and, as the lime- 

 stone rises to the south, the marl and rubble cause a grass- covered 

 slope round the precipitous crest of the cliffs above the Happy Yalley. 

 At Eoft Fach the concretionary band of rubble and thin-bedded 



