EKDLicH.] BASIC VOLCANIC ERUPTIVES MASSIVE BASALT. 247 



posiDg them, leads to this conclusion. Neither of them can be compared 

 to a volcano, although, at first glance, their position might suggest such 

 an idea. They are essentially an accumulation of volcanic material at 

 the points of the original vents. Undoubtedly long periods of time 

 elapsed between the various flows of basaltic lava. Many of the erup- 

 tions producing them may not have brought forth a quantity of lava 

 sufficient to cover much ground. In this case it would have become 

 rigid near the point or line of ejection, thus gradually building up a 

 mountain. 



While the basalts of the entire area show remarkable uniformity of 

 constitution, those from Mount San Antonio, for instance, present many 

 variations. This argues in favor of the view which regards the mouu- 

 tain as one of the points of original outflow. 



For the sake of comparison, a number of descriptions are here repro- 

 duced from the annual report of 1875. Those specimens described from 

 the various stations represent the character of the basalt of the entire 

 area. It will be seen how many varieties were observed on San Antonio, 

 more, perhaps, than could be collected over the whole extent of the ba- 

 salt, excluding points of outflow. 



1. Station 91. Basalt. 



Paste, microcrystalline ; color, middle to dark gray; weathers dark- 

 brown. Contains small spherical cavities, which appear glazed. Brown 

 decomposed inclosures of olivine have a splendent lustre. Magnetite is 

 segregated in small, octahedral crystals. Is altogether very homogene- 

 ous, heavy, and hard. 



2. Station 96. Basalt. 



a. Paste, microcrystalline; color, dark-gray, weathering brown. Oli- 

 vine in exceedingly minute particles. Spherical vesicles, containing 

 small crystals of zeolites; magnetite not visible. 



h. Paste, crystalline ; color, reddish-brown, weathering lighter. Crys- 

 tals of black biotite occur sparingly. Olivine, decomposed, dark, splend- 

 ent brown. Irregular vesicles distributed throughout the entire mass, 

 some of theui containing zeolites. Decomposition of magnetite pro- 

 duces the brown color. 



3. Mount San Antonio. Basalt. 



a. Paste, cryptocrystalline ; color, dark-gray to black. Slightly vesicu- 

 lar; vesicles either spheroid or drawn out; in some of them deposits of 

 zeolites. Olivine the only segregated mineral distinguishable. 



h. Essentially the same as above, but highly vesicular. Vesicles 

 frequently round, while on the same bowlder iu another zone they are 

 drawn out. No mineral distinguishable but olivine, which is partly 

 decomposed. 



c. Paste microcrystalline. Color pitch-black, with fatty lustre. Very 

 compact. Vesicles too minute to be visible. No segregated minerals. 

 Eesembles the typical melaphyrs of Europe. Large percentage of 

 magnetite. This variety is subject to, mainly, three modifications. 



d. Physical character as above, excepting the presence of vesicles ; 

 these are very flat, drawn out to the length of half an inch. Between 

 the larger ones are very minute ones. The rocks break into shaly 

 fragments, owing to the fact that the vesicles have been compressed in. 

 one direction. This latter feature is still more modified in — 



e. Where the compression goes so far as to produce a decided lami- 

 nation. On the surface of fracture, which latter occurs only in the 

 direction of the longitudinal axes of the vesicles, it has an appearance 

 similar to that of the surface of a palm -leaf. The vesicles no longer 



