ENPLicH.] GEOLOGY SAN LUIS DISTRICT, SECTION A. 309 



ish quartzitic limestone, weathering, in heavy strata, very hard and 

 compact, containing numerous crinoid remains, closely resembling the 

 plates of Heterocrimis. Besides these, a few specimens of Orthoceras 

 were found, but in too poor a state of preservation to admit of any 

 specific identification. From these beds, (section Aa,) the next strata, of 

 hard gray limestone, are separated by a thin layer of dark-gray slaty 

 shales, as it is underlying the limestone (&) that formed the top of 

 the plateau upon which station 53 was located. The upper stratum of 

 this limestone contains a number of corals and Spongidce^ and upon it 

 follow thick beds of white, yellow, and pink quartzites (c) weathering 

 into angular fragments, not at all conducive to the comfort of travelers. 

 This pink color seems to be of some importance, as it recurs in several 

 localities, and probably affords a good constant geognostic horizon. Its 

 color is usually a delicate pink, sometimes stained with yellowish or 

 reddish streaks and blotches. Higher up, the character of these quartz- 

 ites changes; they turn into a light-red sandstone, much less compact 

 in texture. With this sandstone I close the Silurian of that locality, as 

 the lithological character of the strata changes radically, and a little 

 higher up Carboniferous fossils are found.* It is obvious that no careful 

 detail-studies of these formations could be made at the time, and there- 

 fore the paleontological part must necessarily appear meager. 



Extending co the south for a distance of seven to eight miles, the 

 Silurian retains very well the character above given. Again, we find 

 all along that the quartzitic limestones rest directly on the granite, 

 forming a sharp, very distinct boundary. But little change is noticeable 

 in the dip of the strata, both as regards direction and angle of dip, 

 which at places rises to 15° to 16°, rarely falling below 10°, however. 

 Station 55 was located on the same strata as station 53, and a compari- 

 son of their features and succession leads to the same result as before. 

 About one mile south of station 55, the Silurian, and with it all the 

 sedimentary beds, suddenly change their course, making an angle of 

 about 90° to the eastward, instead of following their main course in a 

 southerly direction. With this turn the dip changes, swinging around 

 more to the north, so that we find it a little north of east. Probably the 

 cause of its having been changed may be looked for in the eruption of tra- 

 chorheites directly south of the deflected line. The edge of the Silurian, 

 which has narrowed down considerably, is at some places most likely in 

 contact with the trachorheites, although I did not succeed in finding any 

 point where this was distinctly shown. Going eastward, as we approach 

 Badger Creek, we find a patch of coarse-grained granite appearing, 

 which, as usual, directly underlies the Silurian on the west side, while 

 on the opposite it underlies the volcanic rocks. A section carefully 

 taken from station 56, in the direction south 45° west, running for a 

 distance of ten miles, until it reaches the volcanic strata south of the 

 sedimentary, may serve to illustrate the formations here.t 



Proceediug from the southwest, we find the granite partly coarse- 

 grained, partly fine. It is the same patch referred to in discussing the 

 granite of section a as i)orphyri tic granite. A series of yellow to light- 

 brown, hard quartzites are deposited upon it, weathering in tabular 

 fragments. At first these quartzites (Plate IX h) show a steep dip to 

 the northeast of 35°, but it decreases with increasing distance from the 

 granite. Above these strata we find white and pink quartzites of very 



* Tho remainder of the sectioa giveu will be treated of whea speaking of the Carbon- 

 iferous formation. 



tFor convenience only that part containing the sedimentary beds has been takan 

 out. The entire section will be given when speaking of the volcanic rocks. 



