236 



viewed with rays vibrating parallel to these cracks. The interference 

 colours are similar to those of muscovite. The tourmaline occurs in 

 extremely irregular patches and aggregates. It is allotriomorphic. (1) 



Rocks allied to the above, but frequently containing more or less 

 original augite and therefore belonging to the proterobases, occur at 

 various points round the granite-masses of Devon and Cornwall. They 

 have been observed at Waspworthy, Brazen Tor and Cock's Tor near 

 Tavistock ; at the Sanctuaries, near St. Austell ; at Tolcarn, near 

 Penzance, and at St. Ives. 



Where contact metamorphism has alone acted the only internal move- 

 ment which has taken place is that dependent on the destruction of the old 

 and the formation of new minerals in the rock and in veins. The ophitic 

 augite is often represented by ophitic uralite and the ragged masses and 

 skeleton crystals of iron-ore retain their form. There has been no mechan- 

 ical deformation of minerals depending on a plastic deformation of the 

 rock-mass. The rocks of Devon and Cornwall have, however, been pro- 

 foundly affected by earth movements. The sediments have been folded 

 and cleaved and the pre-granitic eruptive rocks have been subjected to the 

 forces which produced the folding and cleavage. The consequent deforma- 

 tion of the rock-masses has been accompanied by interstitial movement 

 and evidences of this remain in their macroscopic and microscopic cha- 

 racters. The most striking macroscopic feature is the development of 

 schistosity and the most striking microscopic feature, apart from the 

 mineral changes, is the development of a more or less definite parallel 

 structure (foliation). It so happens that the regions where the mechanical 

 forces have acted with the greatest intensity are in the immediate neigh- 

 bourhood of great granite masses (e. g., Penzance district) and consequently 

 the two kinds of metamorphism (contact and dynamic) are superposed. 

 This is, in all probability, not a mere accident ; but a consequence of the fact 

 that the intrusion of the granite-mass was connected with the earth-move- 

 ments. It is, however, important to note that the relation of the granite 

 veins, as for example those on the east side of Trewavas Head, to the clay 

 slates is such as to show that the intrusion of the granite must belong to a 

 very late phase of the earth-movement, for the veins traverse highly 

 crumpled and cleaved beds without themselves being in any way affected 

 by the contortions. The intrusion of the granite cannot possibly be 

 regarded as the cause of the crumpling. It seems rather as if it marked 

 the final relief of the intense stress to which the earth's crust had been 

 subjected and brought about a state of equilibrium which, in this district, 

 has not since been seriously disturbed. 



Proterobases and epidiorites occur to a very great extent round the 

 Land's End mass of granite near Penzance, Marazion, St. Just, Gurnard's 



(1) Prof. ROSENBUSCH proposes that a mineral should be said to be idiomorphic when 

 it is bounded by crystalline faces peculiar to itself, and that when, owing to any cause, it is 

 not bounded by characteristic outlines it should be said to be allotriomorphic. Thus in the 

 Tolcarn rock (Fig 1, Plate XVII.) the tourmaline is idiomorphic. 



