364 E. W. Hilgard- Geology of the Delta, 



bogo-ed, tliej can readily be reached almost dry shod at the 

 Northeast Pass. Here, also, the top layers were peeling off 

 in large concave " flakes," from the effect of sun-cracks and 

 drying ; such a surface again overflowed by mud would add 

 another singular feature to the structure of the cones, which 

 may also be noticed in many sections of ancient ones. 



The vent of a large cone in this region (see Plate) formed a 

 basin about 8 inches diameter, containing a puddle of salt water 

 covered with an iridescent ferruginous pellicle, disturbed at long 

 intervals fey small gas bubbles. Evidently, the resistance in the 

 large cone, elevated about 10 feet above the water level, had 

 become too great for the ejection of mud ; in consequence 

 whereof, a lively little cone was glistening and bubbling at a 

 level several feet lower, near the foot of the old cone. But the 

 testimony of the pilots goes distinctly to show, that the active cones 

 become more lively, and dormant cones resume their activity, at high 

 stages of water in the river. At the time of my visit, the water 

 was at an unusually low stage. 



Collapsed Cones.— The cones, especially the larger ones, appear 

 to be unable to survive for any great length of time the cessa- 

 tion of activity. The change that occurs seems to be, m all 

 cases, a sinking of the central portion, often to such an extent 

 that its place becomes occupied by a pool or lagoon, surrounded 

 by a circular rim whose strata incline away from the center at 

 angles invariably much steeper than is found in any recen 

 cones, from 20° to as much as 45°. Outside of this first mn 

 there mostly appears a series of concentric crevasses, sometimes 

 several feet in width and in depth ; and the annular segments 

 thus brought to a level by a subsidence, also exhibit tfie 

 singular feature of a steeper inclination of the lines ot deposi- 

 tion, than is found in any cones now forming. The seeming 

 anomaly of this fact caused me to observe the phenomen 

 closely ; but I have been unable to find an exception to tne 

 rule ; and I have been led to doubt whether in some case, 

 instead of true eruption cones, these collapsed areas ana tn« 

 surroundings may not be the remnants of the original up- 

 heaved "bubble." The objection to this interpretation is tn^ 

 character of the material, which, excepting in a few ^^^^. 

 the Southwest Pass, is altogether unlike the present nv 

 deposits, both in structure and composition. 



A most perfect exemplification of a large central la^^^ 

 (100 feet by 75) surrounded by an elevated ^m and s^^ter. 

 successive, concentric "moats," also in part fi^ed^ ^^ i,. 

 occurs on one of the numerous mudlumps S.W. of St ^^^ 

 Island, on the Southwest Pass (see Plate). The central pon 

 does not always, however, sink out of sight ; sometimes a co 

 mound is still observable, as the center of the more or less cu-o ' 



