204 Professor Sinoluchouski — 0» MecJianics of Overtlirusts. 



EXPLAXATIOX OF PLATE VII. 

 Fig. 



1. Thacops {Trimerocephalus) nnophtludmus, Frecb. Anterior margin slightly 



water-worn. Xatural size. Daymer Bay. [23430.] 



2. Phacops {Trimerocephalus) aff. cryptophthalmus, Emmr. (Giimbel's interpre- 



tation). X 3. Whiteway Farm. [22884.] 



3. Enlarged view of the eye of the same, x 8. 



4. Fhacops {Trimerocephalus) tripai'titus, sp. nov. x 3. "SVhiteway Farm. 



[22884.] 



5. The same, somewhat compressed laterally, x 3. Whiteway Farm. [23431.] 



6. Proehis dunhevidensis, sp. nov. x 2. Old quarry near Landlake, South 



Petherwin. Oi'iginal is in the Southgate Museum, Launcestou. 



7. Froetiis, sp. a. x 2. Whiteway Farm. [23432.] 



8. Proetu.s, sp. b. x 3. Whiteway Farm. [22S84.] 



9. Proetus, sp. c. x 2. Whiteway Farm. [23433.] 



10. Dechenella tcssheri, sp. nov. x 3. Whiteway Farm. [22 88 4.] 



11. P7ti7/y).sJa OTJMor, H. Woodward, x 3. Hestow Farm. [23434.] 



12. Phillipsla cf. minor, H. Woodward. x 3. Showing the glabella and 



ornamented border of head-shield. Hestow Farm. [23435.] 



13. The same, x 2. Showing the pygidium and drawn from a Avax cast of an 



impression. [23434.] 



14. Phxll'qma sp. x 2. Showing the well-sculptured glabella. Hestow Farm. 



[23436.] 

 The originals, unless otherwise stated, are in the Geological Survey Collection. 

 The resrister numbers are g-iven in brackets. 



II. — Some Remauks on the ]Mecii.4.nics of Oterthkusts. 



By M. S. Smoltjchowski, Ph.D., LL.D., Professor of Physics at the University 

 of Lemberg, Austria. 



"E. T. MELLAUD IlEADE evidently wished to elicit, by his note 

 on the mechanics of overthrusts in the Geological Magazine, 

 1908, p. 518, a discussion on these phenomena, as he also tells us 

 in the February K'umber, 1909, p. 75. May I be allowed, therefore, 

 to contribute some remarks on his paper ? 



It is easy enough to calculate the force required to put a block of 

 stone in sliding motion on a plane bed, even if its length and breadth 

 be 100 miles, and I do not think Mr. Mellard Reade meant to use the 

 word 'incalculable' in a literal sense. However great this force may be, 

 it certainly will be easy to mention instances of still greater terrestrial 

 or cosmic forces. Still, I think Mr. Mellard Reade might dispute the 

 analogy with the piling up of the Rev. 0. Eisher's broken ice-sheets, 

 and he could defend Ids statement "that no force applied in any of 

 the mechanical waj's known to us in Nature would move such a mass ".^ 



Let us indicate the length, breadth, and height of the block by a, h, c, 

 its weight per unit volume by to, the coefficient of sliding friction by e ; 

 then, according to well-known physical laws, a force a, b, c, iv, e will be 

 necessary to overcome the friction and to put the block into motion. 

 Now, the pressure exerted by this force would be distributed over the 

 cross-section «, c ; hence the pressure on unit area will be equal to the 

 weight of a column of a height b,e. Putting e = 0-15 (friction of 

 iron on iron), ^ = 100 miles, we get a height of 15 miles, while the 

 breaking stress of granite corresponds to a height of only about 

 2 miles. Thus we may press the block with whatever force we like ; 



1 T. Mellard Eeade, Geol. Mag., November, 1908, p. 518. 



