393 



NOTES ON THE LIZARD ROCKS, 

 (Continued. ) 

 Br THOMA3 CLARK. 



Since my last paper was written, I have continued working 

 on the rocks of this district, seeking for further confirmation of 

 their volcanic origin, and the great changes that are still taking 

 place in them. 



The contrast between the massive gabbros and the horn- 

 blendic gneisses has baffied all former research ; but I believe 

 I am now in possession of materials to bridge over or connect 

 the two extremes, and show their origin and progress from the 

 massive to the banded structure. 



I foreshadowed this in my last paper, but was not then in 

 possession of sufficient materials to fully establish the fact; I 

 have since procured fresh supplies from the quarries, carefully 

 studied the principal cleavage of the rocks, and cut cross-sections 

 thereto ; a course which I often repeated, and thereby arrived at 

 the conclusion that I had discovered the origin and development 

 of the hornblendic structure in some of the gneiss of the Lizard 

 district. Microscopic illustrations of the slides obtained from 

 these specimens I append. 



Coverack. 



Fig. 13, plate D. — From a dark coloured pebble of gabbro 

 selected from the beach at Coverack. It is highly magnetic, 

 and shews what I take to be the origin of the banded structure 

 of these rocks. The olivine is much fractured, and appears to 

 have been the first mineral to succumb to the change, radiated 

 fractures passing from it into the adjoining minerals, as if it acted 

 like numerous charges of low-class explosives. The general 

 direction of the fractures are in the principal cleavage way of 

 the rock, many of the fissures being within -j^ of an inch of 

 each other, and are traceable in many cases (where such thin 

 slices as are necessary for microscopic work would admit) from 

 olivine to olivine, as if within this mineral lay the expansive 

 powers that fissured them : the surrounding minerals in this 

 slide shew more numerous fractures in the felspars than in the 



