26 



NATURE 



[May 14, 1891 



The author has not forgotten the time when he was a 

 beginner, his early failures, and the disappointments of 

 his student-days, when, from the neglect of some slight 

 precaution, he failed to obtain the results he had been 

 led to expect ; and. he has used every means in his power, 

 by minute and specific instruction, to shield those who 

 use his book from similar mishaps. As an instance, take 

 what he says about the effect of acids on minerals. How 

 often has the self-taught man turned wearily to one book 

 after another on mineralogy, in the hope of getting some 

 definite information on this point, and all he arrived at 

 was the curt statement, " Soluble in acids," which each 

 apparently had copied from its predecessor, or all had 

 borrowed from some common source. What acid 1 

 Concentrated or dilute.' Cold or hot.' Quickly, or 

 perhaps only after a fortnight's boiling? All these 

 points he was left to make out for himself as best he 

 could. The happier pupil of Prof. Cole is treated far 

 more liberally, and will not have to weary himself by 

 feeling about in the dark if he attend to the cautions 

 and instructions of the book now before us. The 

 directions for blowpipe-work are equally precise. Only 

 one who has been himself an actual worker would have 

 told the observer to wait " till the first red glow has gone 

 off'' before noting the colour of a borax-bead. Of course, 

 anyone would, sooner or later, find this out for himself ; 

 but, till he had found it out, he would probably blunder 

 not a little ; and anything that economizes time nowadays 

 is not to be despised. There is no need to multiply in- 

 stances ; everyone who uses the book will find that it 

 eminently deserves the epithet of " practical," which the 

 author has assigned to it. 



But are there no weak points on which the critic may 

 exercise his function ? Attention may perhaps be called 

 to the following :— On p. 6, a graphical method, due to 

 Mr. Dalton, is given for determining the full dip of a bed 

 from the dips on two oblique sections. The writer may 

 perhaps be pardoned for preferring a method of his own, 

 given first in the Geological Magazine for 1876, p. 377. 

 But, independently of any personal predilection, it may 

 be said that the diagram in the case of this method is 

 simpler than in that of Mr. Dalton. This makes it easier 

 to recollect, and, besides, the fewer lines there are in a 

 graphical construction the less is the chance of error. 

 In dealing with " streak," it would be well to notice that 

 the true streak of some hard minerals, Iron-glance for 

 instance, is not obtained till they have been rubbed down 

 in an agate mortar. 



Doubt is thrown on the value of Turner's test for 

 the detection of boron (p. 41) : there is an article by 

 Dr. C. Le Neve Foster in the Mineralogical Magazine 

 (vol. i. p. ^-j) which should be consulted in this connection. 



It is hardly worth while criticizing the nomenclature 

 and classification of the crystalline rocks. No two petro- 

 graphers are in agreement here, and probably the exist- 

 ing schemes of arrangement are all of about equal value. 

 There is fortunately no multiplication of species or intro- 

 duction of new names. It might be possible to take 

 objection to the description of Quartz-felsite as a compact 

 form of Granite, for the part played by the quartz in the 

 two rocks is totally different, and must be correlated with 

 a difference in their mode of consolidating. Quartz-fel- 

 sites are specially common as dykes, and there may have 

 NO. I I 24, VOL. 44] 



been facilities for the escape of water in their case, up 

 the fissures which they fill, that were not present in the 

 case of the more thoroughly buried magma of Granite. 

 It was doubtless the presence of water in the granite- 

 magma which kept the quartz fluid or plastic after the 

 other minerals had crystallized ; its escape in the case of 

 Quartz-felsite may have led to the early crystallization of 

 the quartz. In dealing with the foliated rocks, the author 

 touches on the debated point of the " true schists." We 

 are pretty well used to this phrase, and have waited long 

 in the hopes of being told what constitutes a "true schist," 

 but our patience has not yet met with the reward it merits. 

 The author is of opinion that "the alleged distinction 

 between schist-like rocks and schists of pre-Cambrian age 

 requires great delicacy of definition." This is delicately 

 put, and will command the assent of most geologists. 



The pateontological section will perhaps be looked 

 upon somewhat derisively by those well versed in biology. 

 But it will serve its end, which is to enable those who 

 cannot pretend to any large amount of biological know- 

 ledge to know the commoner fossils when they see them, 

 and determine the genus to which they belong. The 

 method may have a large element of " rule-of-thumb " 

 about it, it may be called empirical, but in a large 

 number of cases it is not practicable to attain to anything 

 better. And it has a certain educational value, for it 

 makes a student use his eyes even if it but slightly 

 disciplines his reason. 



That the work deserves its title, that it is full of " aids " 

 and in the highest degree " practical " will be the verdict 

 of all who use it. 



Nor will Dr. Hatch's handy volume be any less 

 welcome. Those who wish to have in a compact form 

 the prominent characters of the rock-forming minerals 

 and the igneous rocks, will find all the information 

 needed by a student concisely and lucidly put forth. 

 Some slight acquaintance with crystallography and the 

 optical properties of minerals is assumed. A short sec- 

 tion on these subjects would have made the book more 

 self-contained, and need not [have increased its size very 

 materially. 



The igneous rocks are defined to be " those that have 

 been formed by the consolidation of molten material." 

 There is a spice of danger in the word "molten," for it 

 may lead to the belief that the fluidity of the material 

 was the result of " dry heat." In the case of a Laccolite 

 the view so generally held is taken, that the overlying 

 beds have been bent up by the intrusion of a molten 

 mass. It is, to say the least, quite as likely that earth- 

 movement caused a differential amount of bending in 

 two adjoining beds, and that, as an empty space was 

 thus gradually formed between the two, the molten matter 

 was driven into it. 



On the subject of the classification of the igneous rocks 

 we find the following healthy expression of opinion : 

 " The various types are so intimately related, that any 

 attempt at rigid and systematic classification is not likely 

 to meet with any great measure of success." Certainly 

 not till some sounder basis of classification than any yet 

 suggested is hit upon. In the meantime Dr. Hatch's 

 grouping is one that from its clearness and simplicity will 

 be a real boon to the student. 



A most useful feature in the book is the list of localities 



