TRANSACTIONS OF SECTION C. 709 



5. The Devon and Cornish Granites. By W. A. E. Ussher. 



From the relations of the stratified rocks to the granites of Devon and Cornwall 

 there is no obtainable evidence as to the upheaval of the latter. 



From evidences of great mechanical disturbance (such as deflections of strike 

 and constrictions of outcrop), and of metamorphism in areas bordering the granites, 

 and from the shapes, relative positions, and internal structure of the granite 

 masses ; from the distribution of the Elvans, and from evidences of the production 

 of cleavage in the area prior to the contact metamorphism of the cleaved rocks, 

 it appears that the sites of the Devon and Cornish granite masses were occupied 

 by the granites or pre-existent and subterraneouslj^ connected rocks of pre-Devonian 

 age, which had, in a rigid state, exercised an obstructive influence on the north 

 and south movements, and had thereby produced great mechanical effects on the 

 surrounding strata prior to the alteration of the latter. 



The contact alteration of the stratified rocks seems to be coeval with the 

 metamorphism of these ancient masses, and the consequent genesis of the granites 

 in their present form during the later stages, or at the close of the Carboniferous 

 epoch. The intrusion of granitoid rocks perhaps accompanied, certainly succeeded, 

 the solidification of the granites, and continued at intervals down to the Permian 

 quartz porphyries. These rocks, called Elvau d^'kes, approximate, with some few 

 exceptions (notably the north and south Elvan of Watergate Bay), to the general 

 strike produced by the north and south movements, and in some cases, as near 

 Camelford, to the main strike deflections produced by the resistance of granite 

 masses to these movements, but in proceeding from granite to killas they ignore 

 the slight uptilt of the latter on the margin of the granites. 



The evidences in favour of the subterranean connection of the Devon and 

 Cornish granites are too strong to be ignored, and this connection annihilates the 

 application of the laccolitic hypothesis advanced by me to account for the relations 

 of the Dartmoor granite, and at the same time contradicts the suggestion of the 

 upheaval of the granites in or through their surroundings. 



6. Malvern Crystallines. By A. Irving, B.Sc, F.G.8.^ 



The author, after ten weeks' field-work upon the Malvern crystallines, has 

 been led, as the result of his observations, to attach a high value to the theoretical 

 S]ieculations of Professor John Phillips, made many years ago, as to the genesis of 

 the rocks for the most part by mineral differentiation from ' one unerupted 

 magma ' ; but he goes further than Phillips did, and agrees with Dr. Callaway 

 that the whole series (with one doubtful exception) is of ' igneous ' origin, though 

 unable to follow that author in recognisiiig a sequence between the diorites and 

 the granites. Reasons for this difference of view are given in the paper. 



The really intrusive rocks are either (1) felsitic dykes (in places a quartz- 

 porphyry), (2) : dykes of basic rock, in many cases some variety of dolerite, though 

 in some perhaps a tine-grained diorite, evidence of their intrusive relation to the 

 granite-diorite series being given in the paper. 



The structural phenomena presented by the rocks of this crystalline chain the 

 author classes as (i) Diorite-gneiss, the primary structure being in some cases 

 rendered somewhat schistose by shearing prior to final consolidation ; (ii) Gneissose 

 (Jiaseriff) structure ; (iii) Phyllolithic structures, upproachbuj in various degrees to 

 true schists, including hornblende-schists (due to local excessive deformation of the 

 diorite by accentuation of pressure against the more rigid felspathic masses), 

 Jissile felspathic rocks (on the cleavage-planes of which secondary mica appears to 

 have "been developed at the expense of the felspar), and sericite-schisfs which be- 

 long probably to a later stage of development, and resemble rather the younger 

 (Casanna) schists of the Alps ; (iv) Shear-planes, in which dynamic action appears 

 to have resulted solely in the degradation of the original minerals after pulverisa- 

 tion unaccompanied by any resultant reconstruction of higher silicates ; (v) Crush- 



' Published in extenso in the Geokigical Magaiine for October 1892. 



