June 3, 1887. J 



SCIENCE. 



537 



pebbles cemented with white limestone, and grad- 

 ualJy changing upward into a firm, barren, homoge- 

 neous limestone. 



This formation was in continuation of, or some- 

 times below, the horizon of the Exogyra arietina 

 marl. Here, then, was the solution of the problem 

 of the San Marcos. The rocks before me were of 

 the later cretaceous, deposited upon the grayel and 

 shingle which had formed the bed of a river during 

 the period of emergence. They had choked up and 

 rendered impervious the suj)erficial layers of the 

 river-bed, but doubtless left the lower gravel and 

 sand beds in as good condition for carrying water as 

 ever. To make the evidence complete, I found, on 

 examination of the rock aa, which lies only a few 

 feet above the river, that it is the soft limestone of 

 the later cretaceous, containing numerous specimens 

 of Gryphaea laeviuscula K. , — a fossil found in great 

 abundance a short distance east and north of Austin, 

 and there occurring at the top of the Austin lime- 

 stone. 



We have, then, the channel, and need only to ac- 

 count for the water to fill it. The Blanco Eiver, in 

 a westerly direction from San Marcos, is about fif- 

 teen miles distant. In the upjDer jjart of its course 

 it is a running stream of considerable volume ; but 



that the whole of the old bed is to some extent per- 

 meated by the waters of the underground river. " 



The extent and direction of this underground 

 channel, and the determination of other streams 

 than the Blanco which may be tapped by it, are 

 promising subjects of future investigation, which I 

 hope at an early date to undertake, not only in the 

 hope of gaining, by a study of the amount of erosion 

 of the older rocks, some idea of the duration of the 

 interval between the two periods of rock formation, 

 but of obtaining some information concerning the 

 fresh-water life of that period. Edwin J. Pond. 



Austin, Tex., May 18. 



Electrical phenomena at the Washington 

 monument. 



In various numbers of Science of recent dates 

 have appeared notices of certain electrical phe- 

 nomena experienced on western mountain-peaks. 

 The peciiliar effects experienced consist in general 

 of a hissing or crackling sound accompanying single 

 discharges, or a continuous flow of sparks, and the 

 characteristic tingling sensation when a finger is 

 presented to any metallic object near by. These ex- 

 periences, despite the common belief, are not rare. 





SECTION OF CEETACEOUS EOCKS AT SAN MARCOS, HATS COUNTY, TEX., LOOKING SOUTH. 



DDD, principal springs ; B, hill upon which is the Chautauqua assembly building ; aa, later cretaceous limestone, with 

 Exogyra laeviuscula E.; bb, Exogyra arietina marl ; cc, firm limestone, with Terebratula Wacoensis E. and Pecten 

 quadricostatus Sowerby ; passing into dd, thin-bedded soft limestone, with Ostrea carinata Law and numerous 

 fossils of types Ostrea, Gryphaea, TurriteUa, Pecten, Cardium, Cypricardia, Trigonia, Tosaster, and Ammonites ; 

 ee, hard but broken limestone, with Caprinas. 



below the point west of San Marcos it loses size 

 rapidly, and at the point where the International and 

 great northern railroad crosses it, and below, it is 

 for the greater part of the year only a dry bed with 

 occasional pools of standing water. 



It has evidently cut through the overlying de- 

 posits, till it has reached the ancient bed of the San 

 Marcos, which, thus filled with water, has been 

 enabled to clear away whatever later deposits lay 

 upon its ancient bed back to the present source of 

 the San Marcos Eiver. 



To a geologist the question would at once occur. 

 Why has not the current opened the whole of the 

 old bed, and so caused the abandonment of the 

 present bed of the Blanco long ago ? The answer 

 lies in the configuration of the older cretaceous 

 strata at its present source. The old river had cut 

 under what was the overhanging cliff of the hard 

 limestone cc, causing it to dip abruptly, as repre- 

 sented in above section, and then found the least 

 resistance in cutting a channel from the softer Ostrea 

 carinata bed rather than in carrying away the fallen 

 mass of the harder limestone. Hence the rocks of 

 the old rive-r-bed proper, at aa, though very soft, 

 are protected from further erosion from beneath by 

 the stratum cc. 



There are, however, small sx^riugs at s, which show 



nor confined to certain persons. At Pike's Peak 

 these electrical manifestations are of frequent oc- 

 currence, and a list has been, published (Report of 

 chief signal officer, 1882, jd. 893) showing the ac- 

 companying meteorological conditions in" fifty-six 

 instances, and proving that these electrical jDhe- 

 nomena are closely connected with the occurrence 

 of hail, snow, and thunder-storms. At these times 

 it is easy to obtain sparks from woollen or fur gar- 

 ments, and to receive shocks on opening the door of 

 the stove, or touching any metallic body. Again, at 

 Fort St. Michael's (IbicL, 1881, p. 768) during the 

 coldest weather of winter, and always after a snow 

 fog, " the air is so electrified that the hair ujpon any 

 loose fur stands up, and a spark can be drawn by 

 presenting a finger to the tip of a single hair." 



In all these cases the observer may be considered 

 as an insulated (perhaps, as in the case of one of your 

 correspondents, he may stand uijon a thick woollen 

 Navajo blanket) body, which, because of the electri- 

 fication of the air, acquires a charge. Contact with 

 a body, in better, although perhaps not very good, 

 connection with the ground, results in a discharge, 

 with the described effects, varying in intensity with 

 the degree of electrification. This condition of 

 things is in part, I think, imitated in some experi- 

 ments I have made at the top of the Washington 



