I) i:\VKi; OF SOUTH TABLE MOUNTAIN. 167 



crowded together like the pebbles in ;i conglomerate. By the aid of the 

 microscope it appears that both matrix and pebble-like masses are made up 

 of angular mineral particles, chiefly glassy feldspar, with some hornblende, 

 biotite, and augite, and also of particles representing the dense groundmass 

 of corresponding andesite. But few true pebbles are present, and these are 

 of the genera] type mentioned as occurring in the beds of .">. The rock is 

 then made ii)) chiefly of particles which are not extreme results of abrasion, 

 and they are plainly derived from a certain type of rock. It is thought 

 entirely appropriate to call it a tuff. The pebble-like masses are to be 

 considered as masses of a tuff probably rounded up by wave action while 

 yet semiplastic. In proof of the correctness of this deduction from their 

 mineral constitution and structure it is to be mentioned that plant stems 

 have been seen in the pebbles, but they are never found extending outward 

 into the surrounding matrix. The lower 8 feet of division 5 are of the 

 character above described, while in the upper in feet there is a gradual 

 transition through sandy clays to a pure clay at top. 



In division 6 there is almost a repetition of the preceding beds with 

 a development of friable sand rock or tuft' in some la vers, of about the 

 composition described. 



Number 7 of the series represents a decided and sudden change in the 

 character of the materials deposited. It is practically a tine conglomerate 

 in which dark and comparatively basic andesites prevail, the lighter ones 

 of the lower horizon beingvery subordinate in quantity. No pebbles >i-r)i 

 in this stratum are over 5 cm. in diameter. This conglomerate nun- be the 

 representative of the thick, dark bed near Castle Rock, but that is hardly 

 probable, as it is here too far below the basalt, and such local development 

 of sandy or gravelly layers is entirely possible at almost any horizon. 



The beds included under 8, 9, and 10 of the section are really not 

 separable except for this particular section. Their differences are such 

 as may arise in any complex of strata at about this general horizon, in 

 different order and manner from that here found. The specially marked 

 conglomerate (7) may also be included in this statement, but the change 

 from ij to 7 is probably always marked by the development of a coarse- 



