3i6 C. N. FENNER 



tallized, are common, but like quartz and calcite, they cannot be 

 given a definite place in the order of deposition. 



The structure which I have described changes toward the surface. 

 It may not have been so well developed in the upper layers, and 

 weathering has obscured the effects. So far have the changes pro- 

 ceeded at this point that the outcrops of rock just above the quarry 

 have to be examined very carefully before any difference in structure 

 can be noted between these rocks and the ordinary firm-textured trap. 

 A wide area of surface surrounding this point plainly shows a survival 

 of thestructural differences described, but immediately back of the 

 quarry they are almost gone, though so well developed in the rock 

 beneath. This point should be noted, for in following on the surface 



Fig. 10 



the area which was underlain by the lake we find occasionally areas of 

 dense-textured rock, lacking practically all evidence of being different 

 from that which overflowed the arid country, and we should perceive 

 that the presence of surface rock of this nature does not necessarily 

 imply the lack of the characteristic structure beneath. In the areas 

 which I have explored, however, the altered rock has been so fre- 

 quently found where it has been expected, and where it should have 

 been developed if the lake had the form assumed from the position 

 of the few well-developed exposures which I first noted and on which 

 I based the. theory of its existence, that its general form and position 

 can be predicated with fair confidence. 



The features which best" survive the effects of weathering are 

 the extremely vesicular or sponge-like facies of the lower layers near 

 the contact, and the smooth, rounded, bowldery forms with brecciated 

 crusts of the upper parts. Of the secondary minerals few survive 

 prolonged weathering, except quartz and prehnite and occasionally 



