Page:Narrative of a survey of the intertropical and western coasts of Australia, Volume 2.djvu/648

 tBOLOGY, J NATUiAI. !!I!'UIt. /do INSTRUCTIONS FOR COLLECI'ING GKOLOGIC,4L ,SPECIMEN8. IT so often happens that specimens sent from distant places, by persons unpractised in geology, fail to give the instruction which is intended, from the want of attention to a few necessary wecantionss that the following directions may perhaps be useful to some of those, into whose hands these pages are likely to fall. It will be sufficient to pre- mise, that two of the principal objects of geological inquiry, ase, to deterniine,--lst, the nature of the materials of which the earth is composed; and, 2ndly, the relative Order in which these materials .are disposed with respect to each 1. Specimens of rocks ought not, in general, to be'taken 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 alter the cha- racters to a considerable distance from the surface.--Petri- factions, 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 conve- nient 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 i

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