HARLECH AND LONGMYND SERIES. 5/ 



adds that these rock-groups, which had been laid down continuously, 

 were subjected to disturbance, the principal effect of which was to 

 throw them into an arch, and to bend over this arch into an isocline 

 (or inverted anticlinal), with a general inclination towards the 

 north-west. The strata likewise underwent a wide-spread foliation, 

 which, in accordance with the structure and composition of the 

 rocks affected, was chiefly developed in certain kinds of material. 

 Subsequent to these changes the south-eastern side of the fold was 

 invaded by the rise of a mass of granite with quartz-porphyries. 

 Accompanying and outlasting this intrusion, a process of meta- 

 morphism went on, the effect of which has been to change fine 

 felsitic tuffs or shales into hard flinty translucent masses, and to 

 superinduce in them a finely-crystalline structure with the develop- 

 ment of porphyritic-felspar crystals and veins and threads of 

 crystalline quartz.^ 



The cathedral of St. Davids is in some parts built of Cambrian 

 sandstone. 



Longmynd Beds. 



These rocks, so named by Sedgwick, from the Longmynd hills 

 in Shropshire, which rise to heights of looo and 1600 feet, consist 

 of green and purple grits, conglomerates, and slates. They are 

 developed at the Longmynd and in the neighbourhood of Shrews- 

 bury, where they are overlaid by Tremadoc Beds. Their thick- 

 ness has been estimated at 8000 feet. At one time they were 

 considered to be the oldest or " Bottom Rocks." Murchison in 

 1834 used the term Longmynd and Gwastaden Rocks, the latter 

 name being taken from a locality in Brecknockshire.^ 



The beds contain Annelide burrows, belonging to two species, 

 according to Mr. Salter, who named them Arenicolites didymus 

 and A. sparsiis. The burrows, as well as ripple-marks, sun-cracks, 

 and rain-prints, have been observed in the beds near Church 

 Stretton, betw^een Caer Caradoc and Stiper Stones in Shropshire.^ 

 Mr. Salter also discovered portions of a Trilobite at Callow Hill, 

 Little Stretton, which he named Palaopyge Ramsayi. 



The Longmynd group is sometimes divided into an upper and a 

 lower portion, the former supposed to be equivalent to the Harlech 

 Grits, and the latter to the Llanberis Slates. But there is no 

 evidence, however, at present to show that the Llanberis Slates are 

 older than the Harlech Grits, for the difference in their lithological 

 characters may easily be explained, as stated by Sir C. Lyell, by 

 looking upon them as deposits of fine mud thrown down in the 

 same sea, on the borders of which the sands of the Harlech Grits 

 were accumulatins:.* 



^ Q- J" xxxix. 324. 2 Proc. G. S. ii. 14. 



^ Q. J. xiii. 192. See also Horizontal Sections, No. 33 (Geol. Survey), and 

 J. D. La Touche, Geology of Shropshire, 1SS4. 

 ^ H. Hicks, P. Geol. Assoc, iii. 102. 



