522 GEORGE DAVIS LOUDERBACK 



the formations mapped as Myrtle, sensu stricto, may be separated 

 into two chief groups or series, each representing quite an extent of 

 geological history, and separated from each other by a distinct inter- 

 val during which there were various intrusions of igneous rocks, and 

 a period of considerable erosion. In other words, the Myrtle forma- 

 tion is a sort of complex, consisting of heterogeneous parts. Its 

 lower division shows more marked variations in characters and 

 geological history, and probably represents the greater period of 

 time. No word short of "series" seems to represent its thickness 

 and variations. The upper part, if we define the top of the Myrtle 

 as the top of the Shasta group, might very properly be called a group,, 

 but if we extend it to include the Chico, it would better also be called 

 a series. 



The lithologic characters of the two divisions differ considerably 

 and will be described first. 



THE LITHOLOGIC CHARACTERS OF THE SEDIMENTARY ROCKS 

 THE LOWER SERIES 



General composition. — The lower series is made up of the following 

 marine deposits in order of their relative abundance: sandstone, 

 shale, conglomerate, chert, and limestone. 



The sandstone. — Sandstone is by far the most abundant rock in 

 this series, and is generally a gray, well-lithified rock, which, on 

 account of its silicious cement, breaks across the grains in such a way 

 that they do not stand in relief on a freshly fractured surface. The 

 material of the grains is of an arkose nature, showing abundant 

 feldspar, etc., and this, combined with its degree of li truncation, 

 makes it sometimes, to the naked eye, have a remarkable likeness 

 to an igneous rock. This may be further augmented by the fact 

 that the sandstones have suffered considerable squeezing and crush- 

 ing, the bedding planes being frequently very difficult to determine, 

 and, if determinable at some point, can generally be traced but a. 

 short distance. Large exposures often occur where the sandstone 

 has a thick-bedded or massive appearance, and the only planes 

 determinable are numerous irregular ones of fracture. Not infre- 

 quently the pressure to which they have been subjected has flattened 

 the grains along definite planes and caused some recrystallization o£ 



