Reviews — Geological Survey of England and Wales. 565 



and sedimentary rocks were formed, and the conclusions arrived at 

 from the observations made in the preceding parts (I. II.) serve to 

 show that the views entertained by Sir H. de la Beche are in the 

 main correct, and represent a vast amount of truth, derived simply 

 from observations in the field. 



These three parts are preceded by some useful remarks on the 

 application of the microscope to petrological research, being the first 

 special petrographical work issued by the Geological Survey. In 

 this introduction the author dwells briefly upon the microscopic 

 characters of a very few of the common minerals which help to 

 form eruptive rocks, and of a few of the most interesting questions 

 relating to those rocks themselves. Besides which, he points out 

 how far the evidence to be derived from a microscopical examina- 

 tion of rocks can be relied on, and how far the older methods of 

 determination formerly, and now, at the field geologist's disposal, 

 can assist, and in some cases rival, the work performed by more 

 elaborate appliances. This introductory essay will be a valuable 

 guide for the student in his investigation of the mineral composition 

 and minute structure of eruptive rocks. 



2. The monograph by Mr. Newton on the Chimaeroid fishes of the 

 British Cretaceous rocks may be considered as a continuation of 

 the description of fossil fishes published in the previous Decades of 

 the Geological Survey, and is the result of a careful investigation 

 of a large series of specimens in the Museum of Practical Geology, 

 the British Museum, and other collections, which are fully acknow- 

 ledged in the preface. This examination has materially increased 

 the knowledge of this group of fishes, and enabled the author to 

 correlate the various parts of certain known species, as well as to 

 establish species for the reception of several new forms, or varieties 

 related to them. As the author remarks, " Every palseontologist will 

 appreciate the difliculty experienced in determining what differences 

 in fossil forms should be regarded as of generic, specific, or varietal 

 value. And bearing in mind the individual variations and the con- 

 sequent gradual evolution of new and distinct forms, which has 

 doubtless taken place in past times, it is obvious, that any lines of 

 division must of necessity be drawn more or less arbitrarily, and 

 although it is found convenient to make such divisions, yet from the 

 very nature of the case they must be to some extent unnatural." In 

 the historical and general remarks, the previous labours of Dr. 

 Buckland are noticed, who first recognized (more than 40 years 

 ago) the Chimaeroid affinities of certain fossil beak-like bodies in 

 the Mesozoic and Tertiary strata. This group was further described 

 in two valuable memoirs, by Sir Philip Egerton (Quart. Journ. 

 Geol. Soc. 1843-47), who established five genera which have since 

 been universally adopted in this country. 



As there has been some discrepancy between the authors who 

 have written about fossil Chimaeroids, in regard to the names given 

 to the different parts, Mr. Newton, in his remarks on the nomen- 

 clature, has explained the terms which are employed in this memoir, 

 and has also given in a tabular form the more important characters 

 of the genera. 



