1853.] RAMSAY LOWER PALAEOZOICS OF N. WALES. 173 



mile to a mile east of the Stiper Stones lies the hilly tract of the 

 Longmynds *, long ago referred by Sir Roderick Murchison to the 

 Cambrian strata. The Geological Survey has adhered to this nomen- 

 clature. No fossils have been found in these rocks. The main mass 

 of the rocks constitute a block of country about 8 or 9 miles long by 

 about 5^ miles wide. A long spur of the same rocks protrudes from 

 the principal block as far as the ancient camp four miles N.E. of 

 Shrewsbury. It is nearly continuous, and about eleven miles long. 

 Between Le Botwood, Shrewsbury, and the Camp it is partly overlaid 

 by the Coal-measures, partly by the Lower New Red Sandstone. On 

 the south the Weniock shale overlies it unconformably, and on the 

 west it is overlaid conformably by the black Silurian shales that un- 

 derlie the Stiper Stones. On the east it is bounded by a great N.E. 

 and S.W. fault that runs from the New Red Sandstone near IJpping- 

 ton (7i miles E.S.E. of Shrewsbury) to the neighbourhood of 

 Gladestry between Builth and Kington. The fault extends about 

 45 miles. South of Church-Stretton the throw is about 2000 feet. 

 West of the fault the rocks of the Longmynd country generally dip 

 W.N.W. at angles varying between 50° and 80°. Contortions are 

 few, and there is no positive appearance of a complete doubling up 

 of any portion of the country with subsequent denudation of the 

 curve. The entire thickness between the Church-Stretton fault and 

 the western boundary, carefully measured (see the Sections of the 

 Geological Survey, sheets 33 and 36), is about 26,000 feet. The 

 superficial area that these rocks occupy is small ; their thickness is 

 as great as or greater than that of any one division of rocks heretofore 

 measured in Britain. The Llandeilo flag-beds with their igneous in- 

 terstratificatious lie on these in perfect conformity, and between the 

 neighbourhood of the Stiper Stones and Chirbury (where the Llan- 

 deilo flags are overlaid unconformably by the Weniock shale) they 

 attain a thickness of about 14,000 feet. The entire thickness of 

 conformable strata between the Church-Stretton fault and the Wen- 

 lock shale at Chirbury is 40,000 feet, and on the west we do not 

 reach the top of the Llandeilo beds, because of the unconformity of 

 the Weniock shale ; while on the east we do not reach the base of the 

 Cambrian strata, because they are cut ofl" by the Church-Stretton 

 fault f. The Longmynd rocks are often hardened, brittle, and flinty 

 in appearance, but they are by no means metamorphosed, even at the 

 bottom, in the sense in which the term " metamorphic " may be ap- 

 plied to the rocks of Anglesea. There is nothing gneissic or foliated 

 in their structure. The old Huttonian doctrine, therefore, that rocks 

 may be expected to be metamorphosed in proportion to their age and 

 the depths to which they have been depressed, is not always a safe 

 guide. The lowest rocks near Church-Stretton have been buried at 

 least four times deeper than the Anglesea rocks were, before the de- 

 position of the Weniock shale, and they are less altered than many 

 of the rocks of Anglesea and Caernarvonshire. 



Roughly estimating the increment of heat in proportion to increase 



* See Geological SuiTey Maps, 60 N.E. and S.E., and 59 N.W. and S.W. 

 t Horizontal Sections of the Geological Survey, sheet 36. 



n2 



