THE ROCKS OF PLYMOUTH. 213 
THE ROCKS OF PLYMOUTH. 
BY R. N. WORTH, F.G.S. 
(Read October 22nd, 1885, and January 21st, 1886.) 
Fottowine up the work begun more than threescore years since 
by the honoured fathers of our local geology, the Rev. R. Hennah 
and Mr. John Prideaux, it has been my privilege from time to 
time to lay before this Society the leading features of the progress 
of geological science in the neighbourhood of Plymouth. 
Since my appointment as Curator of Petrology in our Museum, 
I have been further engaged in endeavouring to bring together a 
complete collection of our local rocks (now numbering some thou- 
sand specimens), and my outdoor work in this direction has been 
supplemented by the examination of the most typical specimens 
acquired, by the methods of modern petrological science. I have 
therefore now to offer the Society some account of the “ Rocks of 
Plymouth.” 
The present methods of petrological investigation are very 
different from those in use when, in 1830, Mr. Prideaux, in his 
“Geological Survey of some parts of the Country near Plymouth,”? 
gave a detailed description of thirty-eight examples of local rocks, 
chiefly drawn from the area between Cocks Tor and the Eddy- 
stone. He had little to guide him but exterior characters; and 
where the separate components of which the crystalline varieties 
were composed could not be distinguished by the naked eye, or 
at most by the hand lens, no other means of certain knowledge 
Was open. 
But it may be asked, “Is no aid to be derived from chemistry ?” 
Not so much as might be imagined, unless the component materials 
of a rock can be isolated. A rock is an association of minerals. 
Definite forms and modes of association give rocks their distinctive 
1 Trans. Plym. Inst. vol. i. pp. 19-44. 
VOL, IX. Q 
