218 Reviews — TJie Geology of Ivyhridge and Modhitry. 



Geology of the Country around Plymoutli and Liskeard " in 1907. 

 The series of formations, stratified and igneous, in the two areas are 

 for the most part the same, but there are certain local differences, and 

 some modifications appear in the tabulation of them in the two 

 memoirs. 



The story of successive attempts to unravel the sequence in the 

 South Devon rocks is well told in the Introduction, and it is 

 noteworthy that the founders of the Devonian system had failed to 

 determine the true sequence in the subdivisions wliich they recognized 

 near Plymouth. Although the Rev. Richard Hennah is only 

 mentioned in the Bibliography, a brief record of his local researches 

 was given in the Plymoutli memoir (p. 2), as "he was the first to 

 point out the presence of organic remains " in the Plymouth limestone. 

 It may be mentioned that he was Chaplain to the Garrison of 

 Plymoutli from 1804 until his death in 1846, and that the loan of his 

 collections of fossils to Sedgwick, Murchison, and Lonsdale "afforded 

 one of the principal sources for determining the relative geological 

 age of the Devonian limestones".^ 



According to Mr. TJssher, the work of Jukes in 1867-8 " is of great 

 stratigraphical value", and "he supplied an entirely new and correct 

 rendering of the stratigraphy of the coast-section south of Plymouth ". 

 Subsequent observers, notably Holl and Champernowne, added much 

 to the knowledge of the strata, but did not succeed in determining 

 the true structural arrangement. 



No one has laboured so long, so earnesth', and so laboriously at the 

 interpretation of this and adjacent areas as the author of the present 

 memoir, nor has anyone more fully realized the difficulties that are 

 encountered. While carrying out the detailed survey on the 6-inch 

 maps during the years 1892-7 he blocked out the main subdivisions 

 in the strata, determined their sequence, and the equivalent rocks in 

 adjacent areas ; but, while surmounting so many of the graver 

 difficulties, he was obliged in some tracts to leave with doubt the 

 precise grouping of the strata. In particular he mentions the 

 uncertainty of the boundary between Middle and Upper Devonian in 

 tlie slate areas, as no fossils distinctive of the lowest beds of the 

 Upper Devonian have been found. Moreover, the higher beds of the 

 Plymouth limestone include portions of Upper Devonian, the zone of 

 Rhynchonella ( Wihonia) cuhoides, as in other areas of South Devon, 

 and no definite plane of demarcation can be fixed between the 

 divisions. Again, in the Lower Devonian the lines drawn between 

 the subdivisions on the map are stated to be often uncertain. The 

 author, however, is in his element when recording in detail the 

 results of his observations in the field and the perplexities with which 

 he had to deal. To anyone following in his footsteps the particulars 

 will be of great service, but they are given at the expense in most 

 cases of more readable, if brief, accounts of the leading lithological 

 and palseontological features of each subdivision. Now that the area 

 around Plymouth has been completed, it may be that the authorities 

 will issue a special colour-printed map of Plymouth, like that of 

 Nottingham and other places, accompanied by a memoir; and these 

 ^ Obituary of Hennah, by L. Horner, in Address to Geol. Soc, 1847- 



