6 



CLASSIFICATION OF ROCKS. 



each bed or stratum of rock is spread universally over the globe, and 

 that a series of beds, in regular succession, environs our planet, like 

 the coats of an onion. This position is, however, much too general, 

 as many beds of rock which are common in one country, are entire- 

 ly wanting in another : but, taken as an illustration of the structure of 

 the crust of our globe over a certain extent, the successive coats of 

 an onion, if they were of different colours, might not unaptly repre- 

 sent the different strata that cover certain districts. 



It may here also be proper to observe, that the different strata 

 which occur under each other, are not arranged in the order of their 

 density or specific gravity. Coal strata, for instance, are often cov- 

 ered with strata of iron-stone, the specific gravity of which is more 

 than twice that of coal. 



I shall now proceed to enumerate the different classes of rocks 

 generally admitted by geologists, and briefly describe the principal 

 characters of each class ; and, in order to direct the attention of the 

 reader more forcibly to the subject, I shall trace on an outline map 

 the principal situations in our own island, where rocks of each class 

 occur, except the recent volcanic. 



All the different rocks and strata that cover the earth's surface may 

 be arranged under the following classes : — 



1. Primary. 



2. Intermediate, or Transition. 



3. Secondary ; comprising 



a. The Lower Secondary Series, and 



b. The Upper Secondary Series.* 



4. Tertiary. 



5. Basaltic and Volcanic. 



6. Diluvial and Alluvial Ground. 



This arrangement is substantially followed by most geologists of 

 the present day, though with some modification of the names. Sev- 

 eral of the French geologists class the lower secondary, including the 

 coal strata, with the intermediate or transition rocks: some urgent 

 reasons may be advanced for this, which we shall subsequently no- 

 tice. Objections have been made to the terms primary, secondary, 

 he, that they do not strictly conform to the present state of geolo- 

 -gy ; but a change of names, which are in general use and well un- 

 derstood, would be attended with no adequate advantage, and would 

 be ill suited to promote the knowledge of the science in an introduc- 

 tory work. It is greatly to be regretted, that a morbid desire to ob- 

 tain celebrity by inventing new nomenclatures, should so much pre- 

 vail among some of the cultivators of natural science. The author 

 is of opinion, that a more simple arrangement of rocks might be made 



* By some geologists these secondary strata are called " the older" and •'' the 

 younger series," terms which ore equally clear and intelligible. 



