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MINERALOGY OF THE ROCKS LYING BETWEEN THE BLACK 

 HEAD AND PORTHALLOW, N.E. OF THE LIZARD DISTRICT. 



Bt THOMAS CLARK, A.M.S. 



To make more complete my paper on " Basal Wrecks and 

 Remnants of Extinct Volcanoes on the S.W. Coast of Cornwall," 

 published in the last Journal of this Society, I purpose referring, 

 briefly, to the mineralogical composition of the rocks between the 

 Black Head and Porthallow, but more particularly to such minerals 

 as illustrate the passage of olivine, &c, into hornblendes. I select this 

 portion of the district for investigation, because the mineralogical 

 transitions or changes there are most recent, its rock-forming 

 minerals are in a better state of preservation for examination, and 

 such as the remainder of the district may be judged by ; and, further, 

 I shall endeavour to show that the Porthallow rocks closely resemble 

 the rocks of other parts of the Lizard district. 



To make clearer my researches, I have selected from a collection 

 twenty-four slides, which I have presented to the Museum of the 

 Institution. Twelve views from these are published with this paper, 

 with particulars appended. Among them are specimens from the 

 South of Coverack, Coverack, North of Coverack, Dean Point, 

 Manacle Point, Porthoustock and Porthallow, illustrating first, 

 where no perceptible change is visible in the rock-forming minerals ; 

 second, where the change commenced ; third, in various stages of 

 progress ; and lastly, where not a vestige of the original forms are 

 perceptible. Perhaps the most interesting and instructive are those 

 where the change is about midway. 



In working out the mineral structure of the rocks, in the before- 

 mentioned district, I started with the southern-most land, and pro- 

 ceeded northward, passing along the brow of the tableland, from 

 whence there is a sharp descent of about 300 ft. to the sea. Along 

 the cliffs and the sharp declivities, the rocks are mostly bare of soil 

 and vegetation, affording good opportunities for collecting specimens. 

 For about two miles the mass of rock appears to consist of an almost 

 horizontal sheet of Lherzolite, or a rock closely corresponding in 



