THEORIES AS TO ORIGIN OF TEPEE CORES. 339 



lated, and was finally abandoned. Its rejection was not founded on a 

 knowledge of the essential nature of the concretionary process, for such 

 knowledge we lack, but on the contrast between the characters of the 

 cores and the characters of concretions, especially those occurring in the 

 same formation and at the same horizon. The concretions are wider 

 than high and have smooth spheroidal forms ; the tepee cores are higher 

 than wide and are externally rough and irregular. The concretions are 

 finegrained and dark ; the cores coarse textured and comparatively pale. 

 The larger concretions are full of crystalline veins, occupying shrinkage 

 cracks ; the cores, though much larger than tlie concretions, are rarely 

 veined. The concretions contain few fossils, and those are chiefly cham- 

 bered shells and Lioceramus ; the cores are composed almost entirely of 

 shells and their fragments, witli Lacina most abundant. 



SPRING THEORY. 



The cylindrical form of the cores suggested that they were built by 

 calcareous springs, for in the Pleistocene lake JNIono lofty towers of tufa 

 were constructed in this way.* This theory was under consideration 

 during the examination of a large number of cores, and special search 

 was made for the concentric structure observed in the Mono deposits. 

 A few features were noted which might belong to such a structure, but 

 in general the search was unsuccessful. Moreover, the horizontal part- 

 ings of shale seem to show that the core was not built up faster than the 

 surrounding muddy bottom. If there were springs their calcareous con- 

 tribution was probably so small that it served only to promote locally 

 the growth of shell-secreting animals. 



COLONY THEORY. 



The al)undance of shells in the cores and their relative scarcity in the 

 surrounding shale and its concretions cannot be explained by assuming 

 a diff'erence in the conditions of preservation. It is, indeed, true that 

 such fossils as are contained in the shale are destroyed in the process of 

 weathering, so that collection is diflicult, but in the unweathered shale 

 they are quite i)erfect, even the nacre being retained. There need, then, 

 be no question tliat their relative abundance in the cores is a })henome- 

 non of original deposition, and the simplcfjt conceivable mode of original 

 deposition is by the direct action of the animals to which tlie shells be- 

 longed. Following this line of thought, we have imagined that each 

 tepee core was the site of a colony of Lucuui, and that tlic remains of 

 each perishing generation furnislied in some way conditions favoral)le to 

 the life of tlie next. The theory reciuires that the conditions be very 

 favorable indeed, for otherwise a colony could not be expected to hold 



•Geological History of Luke Laiiontr\n, >)y I. C. Knssoll. Monot;r!ij)h xi, U. S. ('ieol. Survey, p. 221. 



