242 



NA TURE 



[July 12, 1906 



precautions to prevent experimental errors are often 

 given in considerable detail, as, for example, in regard 

 to freezing-point determinations and conductivity 

 measurements ; yet if this be the intention of these 

 descriptions they are singularly incomplete in other 

 respects. For example, in describing the determina- 

 tion of the freezing point the only thing said about 

 the thermometer to be used is that it may have either 

 a fixed or a variable zero. We venture to think that 

 some description of the Beckmann thermometer and 

 the method of using it would have been of service 

 here. Benjamin Moore. 



THE MAKING OF ROCKS. 

 Pelrogenesis. By Dr. C. Doelter. Pp. xii + 262. 

 (Brunswick : Vieweg und Sohn, 1906.) Price 7 

 marks. 



IN this worlv, which would be valued highly for 

 its references to current literature alone, the 

 author brings together what is known as to the 

 origin of various types of rocks. Its outlook is that 

 of the mineralogist and not of the physical geo- 

 grapher ; but this enables the author, though far too 

 modestly, to bring his researches on the construction 

 of minerals and rocks to bear upon broad geological 

 problems. As a treatise, the book is elementary and 

 yet satisfying ; in the series of which it forms a 

 part, " Sammlung naturwissenschaftlicher und mathe- 

 matischer Monographien," it exactly fills its place 

 as an exposition of prevalent, if not necessarily estab- 

 lished, views. Very often these views are subjected to 

 criticism that shows how far we are from finality and 

 conclusions ; but the lucidity of discussion and absence 

 of bias displayed by Dr. Doelter make us grateful to 

 him as a guide. The history of the struggle for the 

 Rhine in no way affects his scientific judgment; and 

 once again we feel that Austria holds the balance in the 

 geological controversies of our time. 



When we say that the book is elementar\', we 

 mean this in the best of senses. It goes to the root 

 of a question, and compels the reader to understand 

 it. As an example of the large amount of valuable 

 matter that may be compressed into one paragraph, 

 we may take the following (p. So), from a discussion 

 on differentiation in igneous magmas : — 



"Attempts have been made, as we have seen, to 

 connect differentiation fundamentally with the exist- 

 ence of magmas which will not mix with one another. 

 But this is an improbable supposition, since every 

 magma can dissolve any other, as I have shown ex- 

 perimentally. The solubility of one mineral in another 

 depends only on the temperature ; and at a temper- 

 ature varying with each case, the critical temperature 

 of solution, the products of fusion are soluble in one 

 another. Experiment also proves to us that no 

 separation takes place in the fluids so long as they 

 are stirred ; it occurs first as cooling goes on ; where 

 there is no movement, separation can take place ac- 

 cording to specific gravity, even in the fluid state." 



The book opens with a discussion of the causes 

 of fluidity of magmas within the earth, and their 

 occasional appearance at the surface is attributed 

 NO.^1915, VOL. 74] 



primarily to tectonic movements. When relief from 

 pressure comes, the magma becomes fluid, and cor- 

 rodes the surrounding rocks. The gases contained 

 in it operate " like a blowpipe-flame." The results of 

 such corrosion are treated later (p. 116, &c.), and Dr. 

 Doelter remarks, following Daly's recent papers, that 

 basic lavas, coming quickly up broad cracks, reach 

 us in a state of greater purity than acid ones, which 

 move more slowly, and have greater opportunities for 

 affecting the walls that bound them. The acid masses 

 " exhibit traces of the country-rocks, but not neces- 

 sarily near the contact-zone, since, in the case of 

 deep-seated rocks, the absorbed fragments may 

 become distributed in the interior of the mass." 



The author's remarks on the potency of mineralising 

 agents during the consolidation of igneous rocks are 

 based upon his own well-known experiments. Mica 

 thus seems always to require the presence of fluorine. 

 While water is the greatest mineraliser, we are re- 

 minded that we are not dealing with pure water in 

 the earth, but with water containing chlorides, hydro- 

 chloric acid, boric acid, and so forth (p. 24). Certain 

 minerals decompose in their own products of fusion, 

 and give rise to other minerals, or mere glass, on 

 consolidation. In such cases, the crystalline con- 

 dition remains stable only at a lower temperature 

 than that of fusion, and the function of a mineraliser 

 or " crystalliser " is to reduce the temperature at 

 which the substance crystallises out again. If the 

 right point is reached, the original mineral is re- 

 covered in its crystalline form. Thus, in the much- 

 debated case of quartz, the mineral, at ordinary pres- 

 sure, will not separate from its product of fusion at 

 temperatures above 950°. Below tliis temperature its 

 crystals are stable. Above it they are unstable, 

 although their melting-point is not reached until 

 1600° or 1700°. The common minerals that require 

 the help of mineralisers for their formation are albite, 

 orthoclase, quartz, garnet, haiiyne, epidote, wollas- 

 tonite, hornblende, and mica. Hence an acid crystal- 

 line rock cannot arise without mineralisers, and the 

 frequent presence of tourmaline, fluorspar, scheelite, 

 and so forth, in granite, indicating boric acid, fluorine, 

 tungstic acid, chlorides, &c. , bears out in nature the 

 results of synthetic laboratory work. 



On p. 65 it is interestingly pointed out that the 

 different items in the chemical analysis of a rock, as 

 written down, possess very different values, and that 

 too large deductions must not be based on small differ- 

 ences in the quantities of magnesia, soda, or potash 

 stated to be present. Exactitude in these determin- 

 ations is not obtainable with the same degree of 

 success as in the case of silica and alumina, and the 

 alkalies, unfortunately, usually appear as small 

 numbers, in which the second place of decimals be- 

 comes of importance for comparison. The American 

 school, by the by, has made such headway that the 

 word " salisch " slips in naturally on p. 44. 



We cannot dwell on all the important consider- 

 ations here put forward as to the processes that go 

 on during the cooling of igneous rocks. Among 

 these, the description of " Unterkiihlung " on p. 137 

 strikes us as of especial interest. The retention of 



