402 PROF. J. r. I5LAKE ON THE MOXIAN AXD 



into a nearly ^'el•tical position agaiiist the buttress formed b}'' the 

 older Lougmyud rocks, and the directions of the earlier and later 

 pressures have been ypproximately the same. 



4. TheWesteni Boundari/ of the Longmynd Massif luitli the supposed 

 Arclicean Masses. — We now come to consider the -western margin of 

 the Longniyud grits. If the account already given of the rocks of 

 this massif be accepted, and an upward succession broken by an un- 

 conformity be traced all the way from Church Stretton, we are here 

 well up in the Cambrian, as represented by Murchison, Salter, and the 

 Survey, and this western margin has little interest in relation to the 

 older rocks. But Dr. Callaway has here described a " second area of 

 ArchcTcan rocks,'' * and his proofs of their occurrence must therefore 

 be examined. He states that the eastern margin of the Longmynd 

 grits is formed by a fault, indicated on his sketch-map by a straight 

 line ; and immediately to the west of this, and in contact with the 

 fault, come the masses referred to the Archaean. If there were 

 really Archasan exposures here, with the Longmynd rocks dipping 

 away from them, as stated, it is essential to the proof of their age 

 that they should be thus in contact. This, therefore, is the first 

 point to examine. 



In the extreme north of the range is Pontesford Hill. All of 

 this, with the exception of the central dolerite, is referred by 

 Dr. Callaway to the Archaean, the proof assigned being the simi- 

 larity of the rocks to the AVrekin lavas, and particularly that of 

 Lea rock. There is, indeed, a remarkable pyromeride on the north- 

 western border, as noted by Murchison ; but, as we are scarcely 

 justified in assuming that all such rocks, even in the same district, 

 are of the same age, the question must be determined by the strati- 

 graphy. The igneous j^ortion of the hill consists of two masses of 

 acid rock everywhere separated by a mass of basic rock, j^ow this 

 igneous portion is not in contact with the purple grits. These latter 

 are limited to the eastern slopes of Habberley Brook, while the 

 whole of the western slope, which is formed by Pontesford Hill, is 

 occupied by well-bedded, soft, compact pale slate with a moderate 

 dip of about 30° to the west. It is above these slates, on the higher 

 slopes of the hill, that the igneous rocks are met with. On the 

 other, or western, side of the hill only part of the slopes is occu- 

 pied by a spur of decomposed basic rock ; the rest of the ground 

 between the two masses of acid rock shows numerous exposures of 

 pale slates and grits of varying coarseness, with the usual high dip 

 and stiike of the district. As we descend the hill from the doleritic 

 summit we cross the strike of these rocks, and at a lower level come 

 upon the pyromerides. In the neighbouring Lyd's Hole section we 

 obtain further information. Of the section here seen we have had 

 two descriptions, of which that by Sir E. I. Murchison seems to me 

 to render the true interpretation. The slates and grits which here 

 alternate above the falls are very much altered as they approach the 

 igneous rock, the grit being rendered micaceous and the slates 



* Quart, Journ. Geol. Soc. vol. xxxviii. 



