282 SCIENTIFIC APPARATUS. 



with the leading features of the already recognised species and 

 varieties of rocks, to compare the rocks of one district with those 

 of another, and to ascertain how far those he may meet with 

 in the field are referable to known varieties. For the first 

 rough or preliminary examination in the field he carries with him, 

 besides his hammer and surveying instruments, a good lens, a 

 pocket knife, and a small acid bottle. This simple apparatus 

 usually suffices for fixing the rocks with which he meets in their 

 proper place, in one or other of the three classes above referred to, 

 and for identifying them with known rocks. For further and 

 more detailed investigation he may avail himself of ordinary 

 chemical analysis. In the identification of ores and other mine- 

 rals a portable blowpipe apparatus is of great service, but for the 

 study of the minuter structure and composition of rocks the aid of 

 the microscope is indispensable. Thin sections are prepared and 

 mounted in the same way as slices of minerals. Various rock- 

 slicing machines have in recent years been introduced, which are 

 of great service when thin sections cannot otherwise be obtained. 

 Thin slices of rock thus prepared and placed under the microscope, 

 indicate of what minerals a rock consists, how these grew up into 

 the compact mass which they have formed, and what subsequent 

 changes they have undergone. Collections of microscopic prepa 

 rations of this kind are of considerable value, as furnishing a means 

 for the comparison of the rocks of different countries. 



FOSSILS are the remains of plants and animals imbedded in 

 geological formations. They naturally occur almost wholly in the 

 aqueous rocks, just as at the present day it is in the deposits of 

 rivers, lakes, and seas that the remains of plants and animals 

 are entombed and preserved. At first sight it might seem that 

 the only interest likely to attach to these relics of former life 

 would be such as might arise out of the resemblances or differ- 

 ences found to exist between them and the still living plants and 

 animals of the present time. It has been found, however, that 

 fossils, apart from this source of profound interest, are of the most 



