F^ossil Ice Crystals 7 



But it has turned out differently. For some time, I have 

 been called upon occasionally to advise with regard to the 

 finding of the ore in one of our quicksilver mines in West 

 Texas. It is now a well established fact that the distribu- 

 tion of the ore in this mine, and I believe in the entire 

 Terlingua district, bears a definite relation to geological 

 horizons. Successful mining requires search in these 

 horizons. The cinnabar, as it appears, has accumulated in 

 greatest quantity under impervious rocks such as shales 

 and marls along planes that separate these from underlying 

 rocks of more open texture, mostly limestones. The ore has 

 clearly come from below and has risen through fissure 

 planes, which in some cases separate large blocks of the 

 Cretaceous formations. The best ore has been found under 

 the basal part of the Boquillas flags, and under the Del Rio 

 clay in the upper part of the Georgetown limestone. The 

 workings must be so arranged in the mine that these hori- 

 zons can be entered on both sides of a fault fissure. The 

 problem of locating the depth of the desirable horizons in 

 the mine in question would be easy enough, if it were not 

 for the fact that the outcropping rocks consist of a series 

 of sediments with few characteristic fossils. Most of the 

 fossils which occur extend through a range of several hun- 

 dred feet and the beds themselves are quite uniform in 

 character, consisting of alternating thin layers of impure 

 limestones and marls. An attempt was made to correlate 

 the outcropping beds by close examination of the layers ex- 

 posed, but the result was not very satisfactory. A close 

 scrutiny made of each layer on the section resulted, how- 

 ever, in the finding of two features that enabled me to 

 measure the throw of the fault under investigation. Inter- 

 bedded in the Boquillas flags there are some thin layers of 

 bentonite, which are quite persistent and can be followed 

 for several miles. By comparing the distances between 

 these layers and by taking note of their individual thick- 

 ness, it was possible to make a correlation that seemed to be 

 correct. But the proof sought fell just short of being cer- 

 tain. In cases of this kind one always looks for corrobo- 

 rating facts to check one's conclusions. I found this check 

 in the discovery of the layer which carries ice crystal mark- 



