224 



NATURE 



[July 6, 1905 



to the forces which have rolled up the strata and pro- 

 duced such varied superficial and deep-seated pheno- 

 mena. Tliese form the principles of dynamical 

 geology, and it is the description of the resultant 

 structures and the methods of observation which form 

 the chief subject of the interesting handbook just 

 published by Dr. James Geikie under the title 

 "Structural and Field Geology." 



The two first chapters are devoted to an examin- 

 ation of the rock-forming minerals. These are very 

 few in number if we leave out all except those which 

 are the essential constituents of the common rocks. 



After studying their composition and characteristics 

 in hand specimens or small slices adapted for ex- 

 amination under the microscope, there is much to be 



(Reprod 



learnt as to the history of the earth's crust from the 

 observation of large masses of rock. We can see 

 whether they were laid down in comparatively tran- 

 quil water, or hurled along by torrents, or dashed 

 against a shore. We notice that what was once mud 

 or sand or shingle is now solid rock, and we trv to 

 make out in each case whether this was brought about 

 by the introduction of some cementing material or 

 ■caused by the pressure of superincumbent masses, and 

 whether the changes were helped bv the action of the 

 high temperature experienced by rocks depressed to 

 great depths or crushed bv irresistible earth move- 

 ments. Chemical reactions' and the crystallisation of 

 NO. 1862, VOL. 72] 



various minerals out of the material of the rock pro- 

 duce changes on a small scale, as seen in concretions 

 and drusy cavities, or on a large scale as in the 

 case of the formation of vast beds of crystalline lime- 

 stone from the calcareous fragments of various 

 organisms. We can infer from a comparison of 

 certain rocks with the products of recent volcanoes 

 that ancient volcanoes also injected molten matter 

 through the riven rocks, poured out vast sheets of 

 lava, and covered wide areas with volcanic ash. 



The rocks so formed and so altered have yet to be 

 regarded from another point of view. They have been 

 depressed, uplifted, and thrown into all sorts of posi- 

 tions, now being dragged out, now crumpled up into 

 every variety of fold, the compressible portions often 

 making up by vertical thickening what they lose in 

 horizontal extent, and those that would not yield to 

 such molecular re-arrangement being reduced to the 

 same dimensions by crumpling. 



Divisional planes are developed in them, some being 

 due to crush, some to shrinkage, some to the variety 

 in the succession of deposits, and when the strained 

 and bending rock must break it is apt to give along 

 these lines of weakness, so that we find faults 

 commonly coinciding with master joints, thrusts with 

 bedding planes, and so on. 



The last seven chapters are more specially devoted 

 to the second subject in the title of the book, namely, 

 field geology. This could not be altogether dis- 

 sociated from the observations recorded in the earlier 

 part of the work. The information there given is the 

 outcome of original observations in the field, but our 

 author now deals more with the methods employed, 

 and explains what are the most useful appliances for 

 the work and what are the indications which the 

 surveyor must be on the alert to detect. Perhaps, 

 having regard to the numerous monographs which 

 have recently been published on the subject of 

 scenery, he has given greater prominence to the 

 causes than to the effects, to the earth structures to 

 which most scenic features must ultimatelv be re- 

 ferred rather than to the total result of movement 

 and denudation by which anomalous river flows and 

 abnormal features must be explained. 



."V study of recent organisms enables us sometimes 

 to establish the relation between the soft and perish- 

 able parts and the hard parts which alone are 

 commonly preserved in the rocks, and thus by a com- 

 parison of the fossil forms with their nearest recent 

 representatives to learn something of the order of 

 succession of life upon the earth and the conditions 

 under which fossil plants and animals existed. Such 

 analogies must not; however, be pressed too hard. 

 Even such a recent case as the occurrence together of 

 the remains of lion, hyaena, and hippopotamus with 

 the hairy elephant and woolly rhinoceros in our gravel 

 terraces can hardly yet be said to have received an 

 altogether satisfactory explanation. Before we draw 

 inferences from the abundance or rarity of certain 

 fossil organisms we must carefully consider their 

 mode of entombment and the conditions which favour 

 the preservation or the destruction of their remains. 

 _ Dr. Geikie has dealt very shortly with these prin- 

 ciples of palaeontology, but devotes most of his work 

 to the inorganic side of geologv. 



Even with this limitation of subject the work takes 

 a somewhat encvclopadic character owing to the great 

 number and variety of the observations and inferences 

 which have to be recorded. The treatment is rather 

 dermatic than critical. With an author so experi- 

 enced and acute in observation this may be for manv 

 an advantage, but students require a discussion of 

 arguments where conclusions differ, and references to 

 other authors where they mav find the matters more 

 fully treated which are here of necessitv briefly Stated. 



