8 Studies in Geology, No. 3 



two formations are identical in appearance either fresh or 

 weathered, and thus offer every opportunity for confusion ; 

 the shale of the Variegated, however, is always character- 

 ized by the presence of lignite (unit f) and one or two 

 oyster beds above and below the lignite, of which the lower 

 one is fairly persistent, and the other is more or less local 

 in occurrence. Such beds are not know^n to occur in the 

 upper Heath. The identity of the Variegated beds has been 

 proven beyond doubt through observation of its lower 

 contact with the underlying Lower Zorritos formation, and 

 the position of the Lower Zorritos in turn has been deter- 

 mined through tracing it as an unbroken unit to its contact 

 with the underlying Heath. 



It is thus clear that the beds at Alalpaso assigned to the 

 Heath bv Grzybowski are Variegated, and that accordingly 

 the fossils collected by him from that locality, on the basis 

 of which he determined the age of his Heath, are not Heath, 

 but Zorritos in age. The triie Heath formation is not known 

 to bear a molluscan fauna within the Zorritos region. The 

 assignment of Lower Miocene as the age ^ of the Heath 

 formation is not certain ; in the present state of knowledge 

 nothing can be said concerning its age further than the 

 obvious inference that it is older than the Lower Zorritos 

 formation. 



A collection of fossil plants obtained from the Zorritos 

 district in 1875 by C. F. Winslow has been described by 

 Professor Edward W. Berry,® who considered the beds that 

 contain them of Burdigahan age. Mention of this conclu- 

 sion is pertinent at this point on account of the change in its 

 application resulting from the recognition of the Variegated 

 beds at Malpaso, from which general locality the plants 

 were collected. Professor Berry based his stratigraph- 



' Loc. cit., p. 658. 



' Proc. U. S. Xat. Mus.. vol. 55, No. 2270, pp. 279-294, pis. 14-17, 

 1919. 



