158 GEOLOGICAL HISTOEY OP LAKE LAHONTAN. 



Section 5.— ILLUSTRATIONS OF GEOLOGICAL STRUCTURE. 



It is customary to consult the older and usually the more thoroughly 

 consolidated stratified rocks for illustration of geological structure, but as 

 such features are in many cases the records of the manner in which the 

 beds were deposited, it is evident that they should occur in the newest as 

 well as the oldest formations. It is well known that the history of the earth 

 is a continuous record, however fragmentary it may seem at the present 

 day, and that the processes of nature have been the same throughout the 

 geological ages. Nowhere are these axioms more thoroughly sustained 

 than in the recently desiccated lake basins of the Far West. As the gravels 

 and finer sediments deposited in Lake Lahontan afford many instructive 

 records of the circumstances under which they were accumulated, we have 

 prepared the following brief summary of observations relating to geologic 

 structure due to deposition, erosion, etc., believing that it will assist in intei'- 

 preting similar phenomena when observed in older rocks, where they are 

 frequently obscured by metamorphism and other changes. 



STRATIFICATION AND IjAMINATION. 



The sediments forming the upper and lower portions of the Lahontan 

 section consist of fine, homogeneous, evenly-stratified marly-clays, which 

 show a distinct lamination parallel with the planes of bedding. Attention 

 is called to the lamination of these deposits in connection with other features 

 due to deposition, as it has manifestly resulted from the slow accumulation 

 of fine sediments in thin layers, and does not owe its existence to pressure, 

 as is the case in many older rocks. This is evident since both the upper 

 and lower clays are alike laminated, while the higher members of the series 

 at least have never been subjected to the pressure of superimposed deposits. 



CURRENT BEDDING. 



The gravels separating the upper and lower Lahontan clays are char- 

 acterized by extreme irregularity, and aff"ord many illustrations of structure 

 due to deposition. They were deposited in the shallow waters and were 

 much agitated by waves and currents, and among other features present 



