214 Report op the State Geologist. 



dissolved by an acid or otherwise removed. This delicate manipu- 

 lation requires great precaution. Such moulds as can be produced 

 by a foreign substance are sometimes made naturally. Waters 

 charged with mineral substances may deposit these in the 

 place of the carbonate of lime which has been dissolved. The 

 fossil, then, is essentially restored in flint or in oxide of ir n. 

 This result is produced, for example, in calcareous Sponges whose 

 spicules are often converted into silica; also in some MoUusks, 

 Polyps, and particularly in the Brachiopods. 



In these various cases, by dissolving slowly and carefully the 

 surrounding rock by means of a weak acid, splendid preparations 

 may be obtained displaying details which otherwise would have 

 eluded observation. In the Brachiopods, in particular, when the 

 brachial apparatus is silicious, it can be disengaged in this 

 manner. 



An interesting case, and altogether an exceptional one, is that 

 of the preservation of fossils in the amher of the Oligocene 

 period. This substance is resinous, and is secreted by a species 

 of pine (Pinus suGcinifer). It was produced in great abund- 

 ance in such a state of fluidity that it enveloped immense 

 numbers of Insects, Arachnids and Myriopods, which are 

 thus preserved with the minutest details of their organization. 

 Amber forms important deposits in the Baltic provinces, and has 

 been worked there from very ancient times. 



Fossilization of plants. — The fossilization of plants takes 

 place by quite different processes, a fact explained by the differ- 

 ent nature of their tissues. The cells of plants have their mem- 

 branes formed of cellulose, either pure or impregnated with 

 various substances, or even more or less completely changed; 

 but only in very rare cases is it encrusted with calcareous matter 

 or silica (Algse, Equistacea). In the surface of the soil, or in 

 water, these substances decompose, and the plant gradually 

 disappears unless it becomes fossilized. 



Moulds of fossil plants are often found. The plants, when 

 they fall on a soil sufficiently plastic, make an impress there. If 

 the vegetable remains are then removed and a new deposit of 

 sediment a little different takes its place it will give a mould in 

 relief of the object that has disappeared. Yery frequently, on 

 the contrary, the vegetable remains persist while the deposit goes 

 on. In this case a double impression is found, one concave and 

 one in relief. Between the two it may happen that no remnant 

 of the vegetable matter is left, but frequently also it is trans- 

 formed into a blackish substance rich in ulmic acid, which, under 

 the most favorable conditions, preserves the structure of the pre- 

 existing tissues. At other times, if the surrounding rock is porous 

 the vacant space left by the decomposition of the organic matter 

 is fllled by mineral substances dissolved or held in suspension in 

 the waters which have penetrated the interstices ; this is a phe- 

 nomenon identical with that which we have already considered 



