110 DR. CH. CALLAWAY ON THE UNCONFORMITIES BETWEEN 



they are older than the upper part of the Longmyndian, but con- 

 tends that they are not older than the lower part of that series. He 

 accordingly separates the Longmyndian into an upper member 

 (Cambrian) and a lower member (Monian). He furthermore 

 expresses the belief that in one locality (Pontesbury) the felsites 

 regarded by me as pre-Longmyndian are really intrusive in rocks 

 which he considers Cambrian. 



ii. The granitic and gneissic rocks (Malvernian) of the Wrekin 

 and other Salopian localities are older than the Uriconian, since they 

 furnish rounded fragments to Uriconian conglomerates. Prof. Blake, 

 on the other hand, holds that the granites are not pre-Uriconian, 

 for in his opinion they are intrusive in the Uriconian, and he 

 maintains that the (alleged) Uriconian conglomerates are of Cam- 

 brian age. 



In the present paper I uphold my previous conclusions, and offer 

 new evidence in their support. 



II. Beview of the Evidence for the respective Ages of 

 THE Uriconian and Malvernian Eocks. 



In criticising the arguments offered by Prof. Blake, it will not be 

 necessary for me to go into minute detail. If the fundamental 

 positions are shown to be untenable, it would be obviously super- 

 fluous to enter upon non-essentials. My proof is presented under 

 the following heads : — 



1. The Eelsites. 



Prof. Blake regards the Longmyndian as divisible into two groups : 

 the Upper = Cambrian ; the Lower = Monian. Eor the purpose of 

 this section, this alleged subdivision is immaterial, and I shall con- 

 sider the series as one group, the Longmynd Series. 



(a) Cliurch-Stretton Area. — Prof. Blake arrives at the conclusion * 

 " that the volcanic rocks " (my Uriconian) " are younger than the 

 (Longmynd) slates, and have been extruded from their midst." 

 The evidence brought forward to prove this thesis occupies the two 

 preceding pages of his paper ; but after studying these pages very 

 carefully, I cannot find that any distinct proof of intrusion is 

 even alleged. There are certain geognostic difficulties which it is 

 imagined can only be removed by assuming igneous intrusion. 

 Thus, it is alleged that the lower Longmynd slates (" dark shales " 

 of Prof. Blake) show no basal beds, and, therefore, that the volcanic 

 rocks in contact with the slates are probably intrusive.' To this I 

 reply : — (1) The junction between the two groups is a fault f, and, 

 therefore, basal beds are not to be expected. (2) The volcanic 

 rocks are mainly volcanic mud, coarse grit, and lava-flows, which 

 must have been formed at the surface, and could not have behaved 

 like a trap-rock. 



* Quart. Journ. Geol. Soc. vol. xlvi. (1890) p. 407. 

 t See p. 122. 



