Geological Societt/ of London. 87 



structures in this and many other instances led him to infer 

 that they generally indicated that the rock, at a late stage, had 

 consisted of a mixture of previously formed crystalline grains 

 and a viscous magma, that the temperature of the mass has been 

 comparatively low, that it had cooled rather gradually, and that the 

 condition of the magma — i.e. one of very imperfect fluidity — had 

 not permitted of free molecular movements among its constituents. 

 Thus this structure, together with certain others mentioned, might 

 be regarded as indicative of " crystallization under constraint." 



3. " On the Unconformities between the Eock-Systems under- 

 lying the Cambrian Quartzite in Shropshire." By Charles Callaway, 

 D.Sc, F.G.S. 



I. Review of the Evidence for the respective Ages of the Uriconian 



and Malvernian Bodes. 

 The author criticizes the conclusions arrived at by Prof. Blake 

 (Q.J.G.S. 1890, p. 386), and adduces evidence to show : — 



(1) That the felsites regarded by himself as Archsean have not 

 been shown to be intrusive in Longiuynd Rocks. 



(2) That it has not been proved that the Longmynd Series is 

 divisible into two groups, separated by an unconformity ; and that, 

 therefore, the evidence from included fragments remains unaffected. 



(3) That the conglomerates and grits associated with the Uri- 

 conian are an integral part of that system, and are not of Cambrian 

 age ; and that the granitic and metamorphic land-masses from 

 which so many of the fragments in the conglomerates are (ferived 

 are consequently of pre- Uriconian age. 



(4) That the granitic rocks of Shropshire are not intrusive in the 

 Uriconian. 



II. The Belation hetioeen the Uriconian and the Longmyndian. 



Pending the publication of Prof. Lapworth's researches upon the 

 fauna of the Hollybush Sandstone, the author regards it as hardly 

 wise to assign positively a pre-Cambrian age to the Longmyndian 

 system. 



In favour of an actual bi'eak between the Uriconian and Long- 

 myndian rocks, he gives details showing the general discordance of 

 strike between the two groups ; the locally concordant strikes are 

 seen along a line of fault. Furthermore, as the junction between 

 the two groups is faulted, this, whilst of course not proving an 

 unconformity, renders incredible the hypothesis of conformity and 

 igneous intrusion along the line of junction. Again, the conditions 

 of deposit wei-e different : the Uriconian is essentially a volcanic 

 formation, whilst the Longmyndian rocks are characterized by their 

 even sedimentation, and the author maintains that such a change of 

 conditions must indicate a break in time, though the unconformity 

 need not necessarily be very great. 



Lastly, the occurrence of fragments of Malvernian granites and 

 schists (formed at great depths) in the Uriconian Conglomerates 

 indicates the existence of an unconformity between the holocrystal- 

 line and volcanic systems. 



