424 Reviervs — Classijication of Rocks. 



The working petrologist requires nothing heyond the knowledge 

 of such of the properties of the minerals he is likely to meet with, 

 as will enahle him readily to recognize them. The student who 

 knows enough of chemistry or mineralogy to teach him the affinities 

 of any mineral, would never find it in the group where it ought 

 natiurally to be found, and niust refer to the index. "Why not then 

 have at once adopted an alphabetical arrangement ? 



However we may object to the arrangement of the minerals in 

 the first part of Dr. Cotta's work, we could not well make the same 

 objections to his " Classification of Eocks " (Part II.) These are 

 objects of which no natural classification seems possible. They have 

 no definite chemical constitution, or even composition, except in the few 

 cases where they consist of only one mineral, tolerably pure. They 

 seem destitute of any properties or distinct characters of sufiicient 

 importance to mark out any clear natural groups. They run so 

 much one into another, and by such insensible gradations, that 

 whatever genera and species they may be divided into, it seems im- 

 possible to teU absolutely where to place the line of demarcation 

 between one and another, (Take for instances, the Syenite group, 

 p. 176 ; the Itacolumite group, varieties, p. 248 ; the Argillaceous 

 group, pp. 265-26G.) 



The objections to his own, and to all other classifications possible, 

 are weU set forth in Dr. Cotta's introduction to Part II. (p. 124). 



"A scientific classification of rocks is a task of more difficulty than 

 might at first sight appear ; as yet, no one has succeeded in produc- 

 ing a perfectly consistent, and comprehensive system. Not only do 

 the nature of the subject and our own imperfect knowledge present 

 many serious obstacles to consistent arrangement, but in many 

 cases, established usage and nomenclature, too firmly rooted to be 

 lightly disturbed, prevent our changing an old classification, even 

 when based on error. 



Even were our knowledge far more certain than it is, and were 

 we free to overthrow all previous errors and misconceptions, we 

 could not lay down a logically complete system of classification to 

 embrace all rocks, on any principle, whether of okigin. textuke, or 

 COMPOSITION (chemical or mineralogical). We do not find the min- 

 eralogical differences between rocks coincide with those of their 

 chemical composition, nor are either of those dependent on geological 

 position or stratification. There are no rigidly defined classes in 

 nature." 



Dr. Cotta proposes the following general classification for rocks : 



'■'I. — Igneous Eocks (Eruptive Eocks), all of which are most pro- 

 bably products of igneous fusion, 



A. Eocks poor in silica, or basic rocks : — 



(a) Volcanic. — Of which the basalts are the principal repre- 

 sentatives. 



(&) Plutonic. — Of these the principal representatives are the 

 so-called Greenstones (diabase, diorite, etc.). 



B. Eocks rich in silica, or acidic rocks : — 

 (a) Yolcanic, e.g. the trachytes. 



