THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE III. VOL. III. 



No. XI.— NOVEMBER, 1886. 



OBIG-IUAL ARTICLES. 



I. — The Metamorphosis of the Lizard Gabbros. 1 



By J. J. H. Teall, M.A., F.G.S. 



(PLATE XIII.) 



IF we take a general view of the present position of geological 

 science, we are struck by the fact that, although there is sub- 

 stantial agreement amongst geologists on matters relating to the 

 origin of the rocks usually designated as aqueous and igneous, the 

 greatest diversity of opinion prevails with regard to the circum- 

 stances under which the so-called metamorphic rocks have been 

 produced. Every fragment of evidence calculated to throw light 

 on the origin of these rocks, therefore, deserves the most careful 

 consideration. Of recent years special attention has been directed 

 to the effects of mechanical energy in modifying the mineralogical 

 and structural characters of rocks originally formed by aqueous and 

 igneous agencies ; and a suspicion has been aroused that it is in this 

 direction that we must look for a solution of many of the problems 

 connected with the origin of the crystalline schists. A visit to the 

 Lizard Peninsula of Cornwall during the present summer has con- 

 vinced me of the immense importance of this view so- far as that 

 district is concerned. That portion of the peninsula which lies 

 south of a line drawn from Porthalla on the east to Polurrian Cove 

 on the west is formed partly of igneous rocks — such as gabbro, green- 

 stone, serpentine, and granite — and partly of crystalline schists. The 

 igneous rocks, in certain places, become foliated and schistose and 

 sometimes show a definite banding due to a variation in the relative 

 proportions of the different constituents. In other words they pre- 

 sent characters which are usually regarded as distinctive of the 

 crystalline schists. There is, moreover, evidence to show that these 

 characters are mainly the result of a yielding to earth-pressure sub- 

 sequent to the consolidation of the original rock. At the present 

 moment, having just returned from the district, I am unable to treat 

 the subject from a general point of view with any prospect of success ; 

 but it has occurred to me that some details with regard to one of the 

 rocks may not be without interest to members of the Association. 



The gabbros of the Lizard have been noticed by many previous 

 writers, including Dr. Boase, 2 Mr. Majendie, 3 Sir Henry de la Beche, 4 



1 Paper communicated to Section C of the British Association. 



2 Trans. Ge.ol. Society of Cornwall, vol. iv. p. 330. 



3 Ibid. vol. i. p. 36. 



4 Eeport on the Geology of Devon and Cornwall, 1839. 



DECADE III. — VOL. III. — NO. XI. 31 



