24 REVIEWS. 



ment in certain parts of the book. We cannot too earnestly or too 

 frequently give our testimony against this growing vice : we do not think 

 it is endurable, except in a translation ; and an author who condescends to 

 write his text to suit his diagrams is not unlikely to borrow his ideas as 

 well as hi3 woodcuts from others. 



By far the most valuable addition made to this part of the Descriptive 

 Geology is the Table of extinct Genera founded on Mr. Morris' valuable 

 Catalogue of British Fossils. 



In the last three chapters of the Descriptive Geology, Professor 

 Ansted has adopted the Natural method, viz., proceeding from the oldest 

 epochs to the newest ; for, notwithstanding the high authority of Sir Charles 

 Lyell in favour of the opposite course, we consider that Geology, which 

 is the History of the Earth, should be treated like other Histories, and 

 commence with its period of Myths and Fables, its nebular Hypothesis, and 

 Cosmogony, its Azoic Rocks and Crystalline schists, before it enters upon 

 the beaten path of the Secondary and Tertiary Epochs. What should we 

 think of a History of England which commenced with the reign of Queen 

 Victoria and in which we could not read of the good Canute sitting on the 

 shore of the sea in his own chair until we reached the last chapter. 



Of this interesting, because unknown, epoch of the Azoic and immedi- 

 ately subsequent rocks, he writes as follows : 



M 603. Rocks called by Sir R. Murchison and others azoic, as not at present 

 yielding any evidence of life at the time of their formation, are found in Wales 

 occupyhg an intermediate position between crystalline and fossiliferous rocks. 

 These lowest rocks include the Harlech grits and Llanberis slates. They are re- 

 presented in Iteland (on the coast opposite the Isle of Ansjlesea) by similar rocks, 

 in which remains of two species of zoophytes are found. These are hoth referred to 

 the same genus {Oldhamia), and at present th< y are the most ancient forms of 

 organisation that have been determined. 'J he rocks consist of contorted schists, of 

 the kind called by German Geologists grauwacke (anglicised into greywache), fine 

 and coarse grits and fine purple roofing-slates, largely worked in North Wales, altered 

 in places into chloritic and mica schist, and in others into quartz rock. The pre- 

 existing masses out of whose materials these beds were formed have not yet been 

 found in the British Islands, although in Bohemia and in North America there are 

 crystalline rocks, which, from their underlying position, are known to be of still 

 higher antiquity. The thickness of the oldest mechanical rocks in Wales (the 

 Harlech and Llanberis series) is estimated at 1,500 feet. 



41 604. Next in order, and also exhibited in Wales, are the Lingula flags (about 

 2,000 leet thick, containing peculiar fot-sils, and consisting chiefly of gritstones and 

 schists with imperfect slaty cleavage passing into the sandstones and fine quartz 

 rock of the Stiper stones in Shropshire. Above these are the Tremadoc states (1000 

 feet), and above these again the Arenig slates and Arenig porphyries (7,000 leet). 

 In all this great thickness the fossils hitherto found are very sparingly distributed, 

 and the number of species is very small, but a difference is recognised between these 

 species and those of overlying rocks. The Lingula (a small bivalve shell) is the 

 most common fossil ; but there are also two small trilobites, a shrimp-like crusta- 

 cean, and a zoophyte. 



