for collecting Geological Specimens. 



183 



from loose pieces, but from large masses in their native place, 

 or which have recently fallen from their natural situation. 



2. The specimens should consist of the stone unchanged by 

 exposure to the elements, which sometimes alters the charac- 

 ters to a considerable distance from the surface. Petrifactions, 

 however, are often best distinguishable in masses somewhat 

 decomposed; and are thus even rendered visible, in many 

 cases, where no trace of any organized body can be discerned 

 in the recent fracture. 



3. The specimens ought not to be too small. — A convenient 

 size is about three inches square, and about three-quarters of 

 an inch, or less, in thickness. 



4. It seldom happens that large masses, even of the same 

 kind of rock, are uniform throughout any considerable space ; 

 so that the general character is collected, by geologists who 

 examine rocks in their native places, from the average of an 

 extensive surface : — a collection ought therefore to furnish spe- 

 cimens of the most characteristic varieties ; — and the most 

 splendid specimens are, in general, not the most instructive. 

 Where several specimens are taken from the same place, a 

 series of numbers should be added to the note of their locality. 



5. One of the most advantageous situations for obtaining 

 specimens and examining the relations of rocks is in the sec- 

 tions afforded by cliffs on the sea-shore : especially after recent 

 falls of large masses. It commonly happens that the beds thus 

 exposed are more or less inclined ; and in this case, if any of 

 them be inaccessible at a particular point, the decline of the 

 strata will frequently enable the collector to supply himself 

 with the specimens he wishes for, within a short distance. 

 Thus, in the subjoined sketch, which may be supposed to re- 

 present a cliff of considerable height, — the observer being si- 

 tuated at a, the beds b, c, d, though inaccessible at that place, 

 may be examined with ease and security, where they succes- 

 sively come down to the shore at b', c\ and d'. 



6. To examine the interior of an unknown country, more 

 skill and practice are required : the rocks being generally con- 

 cealed by the soil, accumulations of sand, gravel, &c, and by 

 the vegetation of the surface. But the strata are commonly 



disclosed 



