Reviews — Tate's Hudiments of Gealogi/. 131 



more or less liqiiefied state, and at an intense temperature, through, 

 the overlying stratified masses. These rocks are found on examination 

 to contain a considerable quantity of water disseminated through 

 them interstitially or in minute cavities ; and it is obvious what aa 

 amount of internal elasticity or tension must be communicated to 

 such a mass, at its certainly high temperature, by the tendency of 

 these minute particles of water to expand into vapour." 



The theory of pockets or cavities in the earth's crust filled with 

 molten matter at an exceedingly high temperature, is, we believe, 

 open to as grave difficulties as is the theory of a fluid interior. We 

 think, however, it has rather been assumed that geologists do believe 

 in a fluid interior with a moderately thick solid crust. 



Mr. Scrope has himself clearly pointed out, that even highly 

 liquid lava is, when ejected, rather of the condition of thick molasses 

 (solidifying rapidly upon yielding up its molecules of steam), than 

 of a freely-flowing liquid body. 



Any molten matter, therefore, which may exist in the interior of 

 the earth, is doubtless, under such pressure, that even this moderate 

 degree of fluidity is impossible, save where that pressure is locally 

 alleviated by earthquake movements and the opening up of fissures 

 in the crust consequent thereon. How then could tidal movements 

 take place ? 



The strongest argument to our mind in favour of a common source 

 for all volcanic matter — we care not at what depth Sir William 

 Thomson places it beneath the solid crust of the earth — is that 

 brought forward by Mr. David Forbes, F.E.S. (see Geol. Mag., 

 1868, VoL v., p. 97), in reply to the views of Br. T. Sterry Hunt, 

 " That volcanic rocks taken from any quarter of the world, no 

 matter how far distant from one another — from Iceland or Tierra 

 del Fuego, from the Islands of the West Indies, or from those of 

 Polynesia — that in all cases such rocks possess an absolute identity 

 in chemical and mineralogical composition, in physical and optical 

 properties : can any geologist be expected to believe that such rocks 

 have been formed by the melting up of a mere mechanical aggregate 

 of rock-debris, possessing no analogy whatsoever, and whose chemical 

 composition, etc., is known to vary to the widest imaginable extremes." 



The limits of our space preclude us from giving a more detailed 

 notice of the valuable j)refator3'- observations appended to this re- 

 issue of Mr. Poulett-Scrope's "Volcanos"; a work which must 

 always remain the standard book on this branch of geological inves- 

 tigation ; bat we hope in a future number to be able to refer again 

 to some other points touched upon by the learned and accomplished 

 author. 



II. — EuDiMENTART TREATISE ON Geology. [Partly based on 

 Major-Gen. Portlock's Eudiments of Geology.] Part I. — 

 Physical Geology. By Ealph Tate. 8vo., pp. 215. (Lock- 

 wood & Co. 1871.) 



INCE the first edition of Portlock's Treatise on Geology was 

 published twenty years have elapsed. The work was very 



s 



