140 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Se«. 



nized at a few localities so far, but even these remnantal de- 

 posits are of great value as proof of the direct connection of the 

 waters of the Pacific and Atlantic oceans during the final 

 stages of the Middle Eocene and in the Upper Eocene. 



A large number of wells have been drilled in the area between 

 the Panuco and Tuxpam rivers and from such logs as are avail- 

 able, it appears that all the wells which have proved good pro- 

 ducers are drilled into the Tamasopa limestone which is encoun- 

 tered at depths from 1700 to 2400 feet. 



The identity of the Tamasopa is fully proved by fragments 

 of the limestone which have been blown out of the wells, in 

 some of which fragments the rudistes are clearly present. 



The drilling also shows that the Tamasopa, throughout most 

 of this area, is overlain by the San Juan beds, but the irregular 

 thickness of the beds so referable, showing, in place of the 800 

 feet usually attributed to this formation in this area when un- 

 disturbed, only 70 to 150 feet in places and occasionally seem- 

 ing to be missing entirely, indicates that the San Juan was 

 subjected to strong erosion prior to the deposition of the over- 

 lying shales. Since there is no such unconformity between the 

 Papagallos and San Juan anywhere as is found between the 

 limestones and shales in this area, it is evident that these shales 

 are not Papagallos and therefore whatever thickness of Papa- 

 gallos may have originally overlain the San Juan in this region 

 was entirely removed together with a large portion of the San 

 Juan prior to the deposition of the shales now covering them. 



It is probable that a part of this shale belongs to the Chicon- 

 tepec, especially in the western portion of the area, but it is also 

 certain that a large part of it belongs to the Alazan, since 

 samples of the drillings are identical in physical character with 

 the typical shales and at times carry fragments of lamelli- 

 branchs like those of the Alazan. It is also possible that some 

 part of it may belong to the San Rafael. 



Just what part belongs to the Chicontepec and what to the 

 San Rafael is as yet undetermined. 



It is probable that a careful microscopic study of the drillings 

 of the materials overlying the San Juan in connection with 

 similar study of Chicontepec, Alazan and San Rafael sediments 

 would enable us to draw the line between the two formations as 

 found in the wells with some exactness. 



