428 



THE GEOLOGIST. 



to observe the exquisite delicacy of the process by wliicli tliis substitu- 

 tion has been effected, the most delicate characters being as perfectly 

 preserved as in a living specimen. The preservation of the nio.st 

 fragile objects, as delicate shells and stems of corals, is an important 

 fact in reference to the process by which they were originally entombed. 



" The relative positions of the stratified and unstratifiecl masses is 

 such that, as a general rule, the former reposes on the latter. JN'car 

 their surfaces of junction, however, they are frequently dove-tailed, as 

 it were, into each other, by veins of the unstratified mass which pene- 

 trate into the stratified portion. In other cases, vertical (li/kcs rise like 

 walls through the strata ; and, in almost every mountainous district, 

 similar rocks are found either in large irregular masses, or regularly intcr- 

 stratified between the beds of stratified rock. These facts are highly 

 significant as indicating the primitive state of the unstratified mass. 



The positions of the strata of the stratified mass next demand our 

 attention. We have spoken of them as being in general approximately 

 horizontal ; but in numerous instances they deviate more or less from 

 this position, the deviation being more general in the older strata, and 

 usually the greatest in mountainous districts, where the strata become 

 not unfrequentiy nearly vertical. This inclination to the horizon causes 

 each stratum to rise successively to the surface, and thus enables us to 

 ascertain its mineral nature and organic contents. The angle of incli- 

 nation is termed the dij) of the beds. It not unfrequentiy changes from 

 one direction to the opposite, several times within the limits of the same 

 mountainous district, as shown in the annexed diagram, which j ?pve- 

 sents a section in the direction of the dip. Lines through A B 



perpendicular to the section are called anticlinal lines, and a similar lino 

 through C IS called a synclinal line. Such lines will frequently be con- 

 tinuous and nearly straight for many miles, and often correspond more 

 or less with the external character of the district, in some such manner 

 as indicated by the diagram. 



It is this inclined disposition of the strata which enables us to 

 form some rude estimate of the thickness of the whole stratified mass in 

 particular localities. It cannot, in many places, be estimated at less 

 than 20,000 or 30,000 feet, and may possibly be considerably greater 

 It is essential that the geological reader should realize in his own mind 

 the enormous magnitude of the stratified and fossiliferous portion of the 

 earth s crust. 



There is another kind of discontinuity to which these strata 

 are subject, and which may also be best illustrated by a simple diagram. 

 In these cases the whole stratified mass has the appearance of having 

 been dislocated along the nearly vertical planes d e, as indicated by the 

 discontinuous stratum all'c. This dislocation is called a fault. The 



