BULLETIN 39, UNITED STATES NATIONAL MUSEUM. [8] 



frequently occurs in other animal bodies that have become buried in wet or con- 

 stantly damp earth, and packages of pork recovered from old river wrecks have 

 often been found to have undergone j|j|pie same change. 



Every specimen of fossilized man is really only a skeleton, but the wonderful cases 

 of preservation of the human form in the partially hardened volcanic ash of Pompeii 

 are worthy of mention in this connection as illustrating more than one of the facts 

 that have been stated in the foregoing paragraphs. While excavating the buried 

 city the workmen came upon molds of the bodies of persons who were suffocated by, 

 and buried beneath, the shower of ashes from Vesuvius. The bodj', even including 

 the bones, long ago decomposed and removed by the percolation of water which 

 fell from the clouds. Casts of these molds, when discovered, were made by pouring 

 them full of plaster, and when the comparativelj^ soft inclosing matrix was removed 

 an exact counterpart of the body was disclosed just as it fell in death well-nigh two 

 thousand years ago. 



PART II. — APPARATUS AND METHODS OF COLLECTING INVERTE- 

 BRATES AND PLANTS. 



KINDS OF ROCK AND SITUATIONS IN WHICH FOSSILS MAY BE FOUND. 



Sedimentary rocks. — Fossils are never found in granite or in any 

 originally molten rock, while sedimentary or stratified rocks usually 

 contain such remains. All such rocks should therefore be examined 

 for fossils. "A little practice," says Geikie, " will teach the learner 

 that some kinds of sedimentary rocks are much more likely than 

 others to yield organic remains. Limestones, calcareous shales, and 

 clays are often fossiliferous 5 coarse sandstones and conglomerates are 

 seldom so. Yet it will not infrequently be found that rocks which 

 might be expected to contain fossils are barren, while even coarse con- 

 glomerates may, in rare cases, yield the teeth and bones of vertebrates 

 or other durable relics of once living things. The peculiarities of the 

 rocks of each district must, in this respect be discovered by actual 

 careful scrutiny." ^ 



Metamorphosed sedimentary rocTcs. — In mountain ranges sedimentary 

 rocks are often subjected to enormous pressure or to heat in the prox- 

 imity of volcanoes or their outflows of lava. In such places sedimentary 

 rocks are more or less altered or metamorphosed, and the fossils are 

 much changed or distorted, or may be entirely obliterated. "Metamor- 

 phism is a crystalline (usually also a chemical) rearrangement of the 

 constituent materials of a rock. In its production the following condi- 

 tions have been mainly operative: (1) Temperature, from the lowest 

 at which any change is possible up to that of complete fusion; (2) 

 pressure, the potency of the action of heat being, within certain limits, 

 increased with increase of ]3ressure; (3) mechanical movements, which 

 so often have induced molecular rearrangements in rocks; (4) presence 

 of water, usually containing various mineral solutions, whereby chem- 

 ical changes would be effected which would not be possible in dry heat; 

 (5) nature of the materials operated upon, some being much more 



igiR Archibald Geikie, Text-Book of Geology, 3d ed., p. 669, 1893. Macmillan 

 &Co. 



