112 DR. CH. CALLAWAY ON THE UiVCONFORMITIES BETWEEN 



therefore more susceptible of alteration, No. 543 * being within one 

 yard from the Uriconian, and No. 544 about six yards distant. The 

 former is a mudstone, the latter a grit ; and the materials of both 

 are largely volcanic. Neither Prof. Bonney nor myself could detect 

 any signs of alteration or any traces of chiastolite. Indeed, 

 Prof. Blake himself has omitted to give us any microscopic evidence 

 on these points, though it is obvious that they are of critical 

 importance. 



As this crucial case breaks down on examination, I thought it 

 needless to re-examine the other masses of Uriconian which appear 

 on the line of the great Pontesford-Linley fault. 



2. The Alleged Unconfoemities. 



Eoughly speaking, the Longmynd rocks may be divided into an 

 upper member, coloured purple, and a lower member, coloured 

 green ; though purple rocks occur in the lower part, and green 

 rocks are occasionally found in the upper. By Sir E. Murchison and 

 the Geological Survey the two members were united into an un- 

 broken series, and this view has been generally accepted. In 

 Haughmond Hill there is a marked break between the purple and 

 the green beds, as I pointed out to Prof. Blake : but it can be 

 demonstrated that this is due to faulting, and, from a general 

 acquaintance with the Longmyndian rocks, I have not been led ta 

 question the received opinion. 



Prof. Blake is the first to dissent from the old view. He con- 

 siders that the Longmynd Series is divisible into two groups sepa- 

 rated by a marked unconformity. The upper member he correlates 

 with some part of the Cambrian, the lower he places in his so-called 

 " Monian " system. 



I contend, however, that Prof. Blake has not proved his case, and 

 that, if he had done so, he would hardly have refuted my evidence 

 from included fragments, since well-rounded pebbles of volcanie 

 rock are found in the lower series as well as in the so-called 

 "• Cambrian." 



(a) The supjDOsed Break in the Longmyndian. — The general sec- 

 tion t offered by Prof. Blake appears on the face of it improbable. 

 He draws the upper series as dipping at about the same angle and 

 in the same direction as his so-called "Monian"; but, just at the 

 contact, the basement beds of the " Cambrian " are represented as 

 creeping up over the edges of the " Monian," and lying to the east 

 of the junction in outliers, horizontal " Cambrian" resting on nearly 

 vertical " Monian." So extraordinary a section surely requires- 

 some explanation. 



In pursuance of my plan, I have carefully examined one of 

 Prof. Blake's critical sections, the " outlier " at Narnell's Rock, 

 The actual junction is figured by Prof. Blake on page 395 %, and the 



* So numbered in my cabinet. 



t Quart. Jouru. Gaol. Soc. toI. xlvi. (1890) p. 392. 



\ Op. cif. 



