UPPEE CRETACEOUS. 645 



almost wholly granite, the table-lands of the middle are made up of nearly horizontal sandstones 

 and volcanic rocks, while the more northern portions combine the ragged and irregularly dis- 

 posed ridges of the south with occasional flat-topped mountains, capped by rocks of sedimentary 

 or eruptive origin. 



All of the higher ridges of the southern extremity of the Territory are made up of granites 

 and syenites, and formed, during the deposition of the heavy bedded Mesa sandstones, an island 

 of considerable height and very irregular outline. The structure of these mountains is so simple 

 that a further description is unnecessary. It is not until within half a mile south of the mining 

 town of San Antonio that any change in the geology occurs. Here mica slate is encountered for 

 the first time, and forms a belt several miles wide, and running from Todos Santos, on the south- 

 west, past San Antonio and Triunfo, northeast. It probably extends into the Cacachilas Range 

 and forms there, as at the other mining districts, the country rock of the metalliferous veins. 

 Beyond the mica slate again, on the road between Triunfo and La Paz, granite is encountered, 

 making the face of the range and extending to near the latter town. 



In all of the valleys scattered through these mountains and in some of the lone hills on the 

 east side of the peninsula are sedimentary formations of a comparatively late geological age. 

 At Santiago I was informed that 3 miles northeast of that place is a locality where large fossil 

 oysters occur in great abundance, and that they are collected and burnt for lime. I had no 

 opportunity of visiting the locality, a circumstance which I have regretted ever since. A short 

 distance farther northeast, near the coast, at a rancho called Los Martyres, is a high hill of sand- 

 stones, without fossils, dipping to the westward at an angle of about 15°. From its general 

 appearance it is, in all probability, of the same age as the sandstones which make up the mesas 

 above La Paz. In none of these sandstones have I ever succeeded in finding fossils by which to 

 obtain a clue to their geological age. They probably, however, belong to the same group as the 

 Miocene sandstones of Upper California. They have in many respects the same lithological 

 characters and bear the same relations to the granites that those rocks hold where we have had 

 an opportunity of proving their age. Besides this very doubtful testimony, there is stiU another 

 item of evidence which, in the absence of any better, should have some weight. Mr. John 

 Xantus, an able collector, sent from Cape San Lucas to the Academy of Natural Sciences of 

 Philadelphia a few fossil oysters, which, if my memory does not deceive me, belong to a species 

 very characteristic of the Upper Californian Miocene — 0. titan Conrad. Should I be correct, 

 this is important, though half a dozen years is a long interval, particularly if one had never 

 devoted any especial attention to the specimens remembered. 



With so little evidence of then- age, therefore, I have hesitated about pronouncing a decided 

 opinion, preferring to leave it an open question, trusting that some future explorer wiU be 

 more lucky than myself and discover fossils from which these rocks can be assigned to their 

 proper place in the geological scale. In consequence of the difiBiculty I have adopted the 

 provisional name of Mesa sandstone in speaking of the formation. 



After leaving the granitic ranges south of La Paz the whole appearance of the countty 

 changes, and with it the geological structure. The granite itself has disappeared, only to show 

 itself as one or two insignificant outliers, and ia. its place come enormous deposits of sandstones 

 forming flat-topped mountains, ragged and precipitous along the east coast but sloping off so 

 gradually toward the Pacific as to merge insensibly into the broad, low plains of the west. 

 Pretty regularly bordering the west coast and occurring occasionally along the guK are deposits 

 of post-Pliocene age, in places filled with and almost made up of the casts or shells of Mollusca, 

 still living in the adjoining waters. Penetrating both these formations, and often capping one 

 or the other or both indiscriminately, are deposits of volcanic origin. These volcanic rocks 

 usually occur as dikes or broad superficial sheets which have been spread over the top of the 

 mesa subsequent to the deposition of the post-Pliocene, and are by no means uniform either in 

 thickness or in the maimer of their distribution. Very few volcanic cones exist. Almost the 

 only ones are the volcano of the Virgines, north of Moleje, and a series of cones and ridges extend- 

 ing westward to near San Ignacio. Elsewhere the eruptions appear to have taken place in the 



