7 i 4 THE POPULAR SCIENCE MONTHLY. 



named a " geological cheese-taster." It was, indeed, a kind of large 

 cheese-taster, fixed to the end of a long stick. This implement was 

 thrust down, and portions of the subsoil and of the clays or sands 

 beneath were pulled up and examined. Similar devices must obvious- 

 ly suggest themselves according to the nature of the work in different 

 districts and countries. 



In the course of his observations in the field, the geologist will 

 meet with rocks as to the true nature of which he may not be able to 

 satisfy himself at the time. He should in such cases detach a fresh 

 chip from some less weathered part of the mass and examine it fur- 

 ther at home. The detailed methods of investigation, which may be 

 pursued with all the conveniences of a laboratory in town, are not 

 possible to him in the country. But he may subject his specimens to 

 analysis in two cases, and obtain valuable, and perhaps sufficient, in- 

 formation as to their characters. He can easily fit up for himself a 

 small and portable blowpipe-box, a machine for slicing and preparing 

 rocks, minerals, and fossils, for examination under the microscope, 

 and a microscope. 



The blowpipe-box should contain a common blowpipe, platinum- 

 tipped forceps, platinum wire, small bottles with the ordinary re- 

 agents, and as many of the most useful parts of blowpipe apparatus 

 as the space will admit, consistently with the whole box being easily 

 packed into a portmanteau. By means of the blowpipe, it is often 

 possible to determine the nature of a doubtful rock or mineral, and to 

 ascertain the proportion of metal in an ore. A young geologist should 

 take with him to the field only the most essential apparatus and re- 

 agents; he will gradually come to see by practice what additions he 

 may best make to his equipment. 



A convenient and portable form of the rock-slicing machine is sold 

 by Fuess, of Berlin. Where it cannot be obtained, the field-geologist 

 may succeed in preparing his slices by chipping thin splinters from 

 the rock and reducing them upon a grindstone or whetstone. One 

 side of the splinter is to be made as smooth and free from scratches 

 as possible, which can be effected by polishing on a water-of-Ayr stone. 

 This polished side is then cemented with Canada balsam to a piece of 

 plate-glass. When quite firm, the upper side of the stone is ground 

 down until the requisite degree of transparency is obtained. Con- 

 siderable practice may be required, and many preparations may be 

 spoiled, before the observer becomes proficient. But the labor is well 

 bestowed, for in no other way can he obtain the same insight into 

 the internal texture and arrangement of the rocks with which he is 

 dealing. He sees what are the component minerals of a rock, and 

 how they are built up to form the mass in which they occur. He 

 likewise can detect many of the changes which these minerals have 

 undergone, and he thus obtains a clew to some of the metamorphic 

 processes by which the rocks of the earth's crust have been altered. 



