284 APPENDIX. 



that a temperate climate in the arctic regions has throughout geo- 

 logical time been the rule rather than the exception. 



OF FOSSIL PLANTS. 



7? state of preservation of fossil plants has been referred to 

 " ^Mentally in several places in the text; but the following more 

 definite statements may be of service to the reader. 



I. Organic remains imbedded in aqueous deposits may occur in 

 an unchanged condition, or only more or less altered by decay. This 

 is often the case with such enduring substances as bark and wood, 

 and even with leaves, which appear as thin carbonaceous films when 

 the layers containing them are split open. In the more recent de- 

 posits such remains occur little modified, or perhaps only slightly 

 changed by partial decay of their more perishable parts. In the 

 older formations, however, they are usually found in a more or 

 less altered condition, in which their original substance has been 

 wholly or in part changed into coaly, or bituminous, or anthracitic 

 or graphitic matter, so that leaves are sometimes represented by stains 

 of graphite, as if drawn on stone with a lead-pencil. Yet even in 

 this case some portion of the original substance remains, and without 

 any introduction of foreign material. 



II. On the other hand, such remains are often mineralised by the 

 filling of their pores or the replacement of their tissues with mineral 

 matter, so that they become hard and stony, and sometimes retain 

 little or nothing of their original substance. The more important 

 of these changes, in so far as they affect fossil plants, may be ar- 

 ranged under the following heads : 



(a) Infiltration of mineral matter which has penetrated the pores 

 of the fossil in a state of solution. Thus the pores of fossil wood 

 are often filled with calcite, quartz, oxide of iron, or sulphide of iron, 

 while the woody walls of the cells and vessels remain in a carbonised 

 state, or converted into coaly matter. When wood is preserved in 

 this way it has a hard and stony aspect ; but we can sometimes dis- 

 solve away the mineral matter, and restore the vegetable tissue to a 

 condition resembling that before mineralisation. This is especially 

 the case when calcite is the mineralising substance. We sometimes 

 find, on microscopic examination, that even cavities so small as those 

 of vegetable cells and vessels have been filled with successive coats 

 of different kinds of mineral matter. 



(b) Organic matters may be entirely replaced by mineral sub- 

 stances. In this case the cavities and pores have been first filled, 



