22 Ot 01 I li I [H3BOLI 



:cd surface. Most rocks yield so HUM h to t lie- 

 action of the weather as to acquire a decomposed, 

 crumbling crust, by which the true colour, u \tun 

 composition of the rock itself may he entirely concealed. 

 rocks, of which the outer nil.ir. may 



differ greatly from each other in essential <har. 



;. two rocks may assume a very different aspect 

 externally, and yet may show an identity of composition 

 on a freshly -fractured internal surface. The hammer, 

 therefore, is required to detach this outer deceptive crust. 

 If heavy enough to do this, it is sufficient for the pur: 

 any additional weight is unnecessary and burdensome. 

 A hammer, of which the head weighs one pound or a fr\\ 

 ounces more, is quite massive enough for all the ordinary 

 requirements of the field-geologist When he proceeds 

 to collect specimens he needs a hammer of two or three 

 pounds, or even more, in weight, and a small, light 

 chipping hammer, to trim the specimens and reduce 

 them in bulk, without running a too frequent risk of 

 shattering them to pieces. 



Hardly any two geologists agree as to the best shape 

 of hammer ; much evidently depending upon the indi- 

 vidual style in which each observer wields his tool. This 

 (Fig. i) is the form which, after long experience, we have 

 found in the Geological Survey to be on the whole the 

 best A hammer formed after this pattern combines, as 

 may be observed, the uses both of a hammer and a < 

 With the broad, heavy, or square end, we can break off a 

 fragment large enough to show the internal grain of a 

 rock. With the thin, wedge-shaped, or chisel -like end, 

 we can split open shales, sandstones, schists, and other 



