70 DIVISIONS OF GEOLOGICAL WORK. 



Petrological investigations lead us to arrange the rock masses of 

 the globe into these two classes : 



ist Class. zd Class. 



Crystalline. Non-crystalline 



Unstratified. Stratified. 



Unfossiliferous. Fossiliferous. 



Origin. By all these characters, separately and comparatively con 

 sidered, the two great divisions of materials which compose the 

 external parts of our glohe are proved to have been produced by 

 entirely opposite causes. Stratified rocks are analogous to the modern 

 products of water, and were therefore called by the older authors 

 Neptunian, while unstratified rocks are analogous to the modern 

 products of volcanoes, and receive the names of Plutonic and Vol- 

 canic, according to the conditions under which they cooled. 



Mode of Study. The distinction now insisted upon between rocks 

 of deposition and rocks of eruption or non-crystalline and crystalline 

 rocks, is of the highest importance, and requires the closest attention 

 at the very commencement of the study of geology. For not only are 

 these different classes of rocks distinguished by most important 

 general characters, but even the methods by which they are to be 

 investigated, and the preliminary knowledge required for this purpose, 

 are entirely distinct. Amongst the stratified rocks a knowledge of 

 zoology and botany or biology is required to understand and develop 

 the past history of the remains of plants and animals, which were 

 buried at successive periods ; on the contrary, among the mountains 

 associated with granite and metamorphic rocks, where minerals of 

 every hue and form appear in ever-different combination, scientific 

 mineralogy is of much higher importance, and study of slices of rock 

 under the microscope is often necessary. 



In consequence, geology divides itself into two branches one, 

 Biological, which links itself with the natural history of modern plants 

 and animals ; and the other, Physical, closely connected with chemistry 

 and natural philosophy. And we have now, and have always had, 

 two distinct groups of geologists, whose progress and discoveries have 

 been as different as the preliminary knowledge which their different 

 spheres of research required. 



A geologist of adequate attainments must now indeed be acquainted, 

 at least generally, with both branches of this wide subject; and there- 

 fore he who is unacquainted with either mineralogy on the one hand, 

 or zoology and botany on the other, must be considered as only 

 half-prepared for original investigation. He must be further instructed 

 in palaeontology and physical science before he can be sent to explore 

 an unknown region, or permitted to give an opinion on the whole 

 theory of geology. 



As much practical knowledge, therefore, as can be easily gained of 

 the minerals which enter most frequently into the composition of 

 rocks and veins, and of the natural history of the plants and animals 

 whose remains lie buried in the strata, is absolutely necessary to 

 the student's progress in this science. 





