94 Observations Concerninsc Fossil Organic Remains 



■Is 



the formation of plastic clay, of molasse and conglomerates 

 of various kinds, intermediate between these strata ? In eve- 

 ry formation, from the lowest to that which is uppermost, we 

 perceive a repetition of rocks and strata, whose chemical 

 composition is similar ; that is, siliceous, argillaceous or cal- 

 careous. But the difl'erence in structure is most obvious. 

 The lower formations are crystallized and firm ; whilst the 

 upper are earthy and loose. The intermediate formations 

 difl'er from these, as well as from each other, in a manner no 

 less remarkable. A striking character of the transition for- 

 mation, and which separates it from the primitive and secon- 

 dary (more properly " tertiary") is seen in the alternation of 

 a series of rocks, in which there is a regular recurrence of 

 similar beds, rendering perfectly obvious, likewise, the limits 

 of this extensive class of rocks. Another characteristic of 

 the same formation exists in the three great formations which 

 hold nearly the same rank in different parts of the globe ; 

 They are, 1st, talcose granular limestone, graywacke with 

 anthracite and mica slate ; 2dly, syenite and porphyry with 

 crystals of hornblende, and occasionally quartz ; 3dly, clay- 

 slate, graywacke and black limestone. 



The mica slate with anthracite and clay slate serves as the 

 connecting points of the transition with the primitive ; whilst 

 on the other hand, the fine grained graywackes and por- 

 phyries, abounding in the crystals of quartz cause it to ap- 

 proximate to the secondary. 



The different ages of these formations are likewise indica- 

 ted by their position. For example, the porphyries of Guan- 

 axuato repose upon a bituminous clay-slate, those of Hun- 

 gary upon a transition mica slate, whilst those of the Andes, 

 of Q.uito are situated upon primitive rocks ; large masses 

 of graywacke are to be seen in the oldest slate rocks of the 

 transition class; whilst extensive beds of graywacke are met 

 with, whose origin is more recent. In fine the transition 

 class of rocks is distinguishable both by structure and age 

 from the primitive, — the limits of the latter are in general 

 simple ; while those of the former are more complex. 



When we observe the highly uniform and decidedly crys- 

 talline structure in the primitive rocks ; when we view this 

 character much less distinctly in the transition ; still less in 

 the secondary ; and not at all in the tertiary ; is it probable 

 that a geologist could be found, who would maintain that 

 they were, nevertheless, all formed at the same epoch ? 



