FALSE STRATIFICATION. 167 



tification. But we might also consider the deposit a as unconform- 

 able, and the deposit c as conformable, from regarding the parallel 

 joints., i to k, as those of stratification ; and it is also possible to 

 view both a and c as unconformable relatively to b, by considering 

 the other joints as those of the strata. It may be often difficult to 

 decide ; nevertheless, in general, the schistose division is frequently 

 a structure which has perhaps a certain crystallization of mica- 

 ceous matter ; and it is this character, therefore, among others, that 

 we must ordinarily select. Now, the joints of dislocation, for one 

 ci the other division must have been thus produced, are splits 

 united and well marked, often a little open, which are ordinarily 

 prolonged into several consecutive deposits, 

 while the joints of stratification are more un- j 

 dulated and more adherent. The most irregu- 

 lar undulations of true strata are often tra- 

 versed throughout by the schistose structure 

 (Jig. 303), without alteration. This circum- 

 stance evidently shows that this structure is 

 an effect posterior to the contortion of beds, 

 and may be attributed to a metamorphism, 

 more modern than their derangement. The 

 extraordinary divisions just mentioned, are 

 sometimes termed false st ratification. 



18. Organic remains, which are very numerous in most sedi- 

 mentary deposits, also furnish a means of recognising strata. 

 There are some which are peculiar to certain deposits, and are 

 not found elsewhere, and which are therefore distinguished as 

 geo^nostic horizons. Thus, the Silurian or Devonian formations 

 are clearly recognised by the presence of the remains of a cer- 

 tain family of crusta'ceans, named trilobites (Jig. 4, p. 28). The 

 Gry'phea arcua'ta (fig. 71, p. 55), is found in the has, and 

 only in it : the ex'ogy'ra vir'gula (fig- 109, p. 65), belongs to 

 the upper part of the jura'ssic formation ; baculi'ies (Jig. 130), and 

 turrili'tes (fig. 131, p. 72), begin and end in the creta'ceous period. 



19. Although the remains of mollusks and small animals aro 

 found entire, and therefore easily recognised, those of large mam- 

 mals, &c., often exist only in fragments ; and, without the nece 5- 

 sary knowledge, the family, genus, or species, could not be dis- 

 covered. But those well acquainted with comparative anatomy, 

 and the laws which govern in the organization of animals, can 

 deduce the form, and even the habits of an animal, often from a 

 Dingle bone. 



" Every organized being may be considered as an entire and perfect sys- 



18. How do organic remains assist us in distinguishing the relative age 

 of strata ? 



19. How is it that a portion of the fossil remains of an animal enable ua 

 to recognise its class ? 



37 



