OF THE KIO-TI^'TO MINES. ZOO 



with regard to the relative ages of the successive ferruginous lake- 

 deposits. Greatly as they differ in altitude, it is probable that their 

 difference in age would not exceed a few thousands of years. Un- 

 fortunately, however, we have no fossil evidence on this point, since 

 fossils have only as yet been found in the one deposit of the Mesa 

 de los Pinos. 



EXPLANATION OF PLATE VI. 



Fig. 1. Map of the Eio-Tiuto mining district. Scale ^^^oo- 



2. Section across the Rio-Tinto mining district. Scale ^to^ott- 



Discussion. 



Mr. P. Fowler inquired why Mr. Collins attributed the iron- 

 ore forming the outcrop of the pyrites-vein-deposits to the action 

 of lakes. He believed they were due to decomposition of pyrites. 

 With regard to the question whether the deposits of pyrites are to 

 be characterized as veins or masses, he believed them to be veins, 

 as they never cut out in depth ; they often narrow, but open out 

 again. Mr. Collins had omitted to notice that all the veins crop 

 out in great depressions, due, probably, to enormous faults. The 

 greater the depression the greater the width of the vein. 



Mr. KiTTO agreed with Mr. Collins as to the iron-ore deposits 

 being lacustrine in their origin. They were stratified, and contain 

 the remains of plants. He also was inclined to class the masses of 

 pyrites at Eio Tinta as veins. 



