Lesley,] 



28 



[Jan. 2 and Feb 6. 



which in part crystallized in rhombohedral crystals, the forms of which 

 we now see, in the outcrops, emptied by dissolution. The silica hardened 

 (without crystallizing) around these rhombs, so that we see the same 

 cavities in it. The iron became peroxydised as fibrous hematite and the 

 silica can be obtained by dilute nitric acid also in the same fibrous form. 

 All this points to the first formation of the iron ore while the rocks were 

 still at a great depth, wet and soft and warm. 



But at the end of the coal era the Middle States rose from the waves 

 and have never been covered by the ocean since that time. The edges of 

 the Bellefonte Fault stood as a mountain range as high as the Alps (see 

 Fig. 4), and the backs of some of the great anticlinals of Pennsylvania 

 must have formed plateaus then as high as Thibet and Bootan are now. 



Fig. 5. 



Erosion commenced and has continued through the Permian, Jurassic, 

 Cretaceous and Tertiary Aj^es to the present day, and still goes on. The 

 high plateau was gradually worn down to . the present surface. Moun- 

 tains once 30,000 or 40,000 feet high are now but 2,000 or 3,000 above 

 sea level. The valleys were excavated as the mountains lowered, and 

 the outcrops of the Lower Silurian limestones of Nittany Valley are but 

 800 to 1300 feet above tide (see the contour lines of the map). 



This slow erosion gives us the second part of our explanation of the 

 brown hematite iron ores. It explains the innumerable caverns and sink 

 holes and dry hollows of this Nittany and other limestone valleys. It 

 leads us to expect to find traces of such caverns and widened fissures 

 and sink holes of the last preceding age, filled up with a wash of clay, 

 sand, and iron ore from outcrops lately existing not far above the out- 

 crops which run along the present surface. 



The erosion now still going on, and the special activity of the last 



