384 Marcous Geological Map of the United States. 



the Geological Society of France, presents us, in addition to the 

 geology of the Atlantic States, a view of the geology of the broad 

 and comparatively unknown region between the Mississippi and 

 the Pacific. Eepresenting regions which have not been visited 

 by the person making it, such a map is necessarily a work of 

 compilation, inference and generalization, and in the present 

 state of our knowledge, some errors are to be expected. I will 

 not undertake to say how far the author has faithfully used 

 the means in his power for making a good geological map, but 

 as there are errors too important to pass unnoticed, I will simply 

 point out those which are most glaring and most likely to mislead 

 foreign geologists. I shall confine myself solely to the western 

 part beyond the Mississippi.* 



Commencing on the Pacific coast, the peninsula of San Fran- 

 cisco is represented as composed of erupted and metamorphic 

 rocks, being colored the same as the Sierra Nevada and Appa- 

 lachians. The rocks of that peninsula, and on both sides of the 

 Golden Gate, are chiefly sandstone and shale, and the same for- 

 mation extends along the shores of the Bay to and beyond San 

 Jose. Not only the extent and position, but the lithological 

 characters of these rocks are discussed in a published report, f 

 which was in the hands of the author of the map previous to 

 its publication. The representation of the granitic rocks is not 

 confined to the end of the peninsula, but is continued southward 

 to the western shores of the Tulare lakes where the formations 

 are chiefly miocene tertiary, the eruptive rocks scarcely ap- 

 pearing. 



The promontory called Point Pinos, which forms the headland 

 of the Bay of Monterey, is represented as tertiary, while a por- 

 phyritic granite constitutes the whole point and forms the coast- 

 line south to the Bay of San Carlos, and is probably continuous 

 southward to San Luis Obispo ; forming a high and unbroken 

 line of coast, all of which is colored tertiary on the map. Cast- 

 ing the eye further south, we find the color denoting the erup- 

 tive and metamorphic rocks again usurping the place which 

 should be colored tertiary, at Point Conception, which consists 

 of beds of conglomerate and sandstone. 



The broad alluvial tract at the head of the Gulf of California 

 — the Colorado desert — is made to extend nearly due north and 

 parallel with the Colorado to the Soda Lake. The published 

 description of this valley gives its direction as northwest and 

 southeast, extending to the foot of San Bernardine Mountain. 



* A former map by M. Marcou, published at Boston a little over two years since, 

 was reviewed in vol. xvii, of this Journal. The present map is in part open to the 

 same criticisms. 



f Preliminary Geological Report on the Pacific Railroad Route, surveyed by Lieut, 

 R. S. Williamson in California, House Doc, 129, Washington, D. C, Jan. 1855. 



