272 licvicics — Geological Surveij Memoirs. 



which is stated by jVTr. lleid to correspond with no internal structure 

 of the rock, and is probably "due to some unrecognised organic 

 agency." 



In the chapter on Igneous rocks there is a description of the Gneiss 

 of the Eddystone by Mr. H. H. Thomas, while notes ou the Basalt, 

 Dolerite, Felsitic Rocks, and Quartz-jjorj^hyry are contributed by 

 Dr. Teall. There are short chapters ou the superficial deposits and 

 economics. 



In his "Geology of the Country around Plymoutli and Liskeard " 

 Mr. Usshcr points out that " the structure of the Devonian rocks of 

 Cornwall, to which the area comprised in Sheet 348 furnishes the 

 key, remained practically unknown until the year 1890." This topic 

 was discussed by him during the excursion of the Geologists' Associ- 

 ation to Plymoiith at Easter, 1907. 



In a region so complicated it is not surprising that it has taken 

 many years to establish the main structure. Beyond marking in the 

 Plymouth limestone and the various eruptive rocks, De la Beche made 

 no attempt to depict the various types of stratified rock which he 

 recognized in the Devonian area in the course of his comparatively 

 rapid survey on the old one-inch maps ; but, as remarked by Dr. Teall 

 in the preface to this memoir, De la Beche's "facts were noted with 

 such precision that the present author, from experience he had gained 

 in the Devonian area of South Devon, was enabled so to interpret 

 De la Beche's observations as to be able to give in 1890 a forecast of 

 the probable structure in the Devonian rocks of Cornwall." Single- 

 handed, Mr. Ussher has been occupied in the detailed mapping of the 

 Devonian rocks from Torquay to Liskeard and St. Austell for a period 

 extending over more than thirty years (though occupied elsewhere 

 during portions of the time), but while gradually establishing the 

 sequence and age of the main rock-divisions it was not imtil 1900 

 that he had satisfied himself with regard to the relative position of 

 the Looe Beds and the Dartmouth Slates. 



The present memoir embodies the detailed information on which 

 the new geological map is based, and will be an invaluable guide to 

 all who devote themselves to further research. 



To the reader who has no personal knowledge of the district a 

 summary of the leading characters and fossils of each division would 

 have been a useful addition to the account of the general geology and 

 structure given in the Introduction. 



The Culm Measures appear nearly everywhere to be in faulted 

 relation with the Devonian ; but in some places Middle Culm Measures 

 rest, apparently with marked unconformitj', on Upper Devonian Slates. 



There is a short chapter on evidences of ]^ew Red rocks, a brief 

 reference to the possible Eocene deposit on the Hoe described by 

 R. jS". Worth, and descriptions of the various superficial accumulations 

 and economics. The petrographical notes by Dr. Flett include accounts 

 of the schalsteins, spilites, proterobases, diabases, and picrites, among 

 the last-named being the well-known rock of Clicker Tor. 



In the memoir on the Quantock Hills Mr. Ussher records observa- 

 tions for the most part made many years ago when the area was 

 resurveyed on the old one-inch map, but these are supplemented by" 



