T. Mellard Reade — Mechanics of Overthrusts. 75 



obtained from Slaty Creek and its branches, -where they occur in 

 a carbonaceous argillite, the outcrop bending round on itself in a horse- 

 shoe form of which the longer or westerly limb has hitherto been 

 unexplored. Nor have any fossils been as yet obtained from the 

 extreme southward extension of this limb. In some parts the replace- 

 ment of the chitin of the fossils by pyrite is somewhat common, 

 rendering the structure obscure. 



Dr. Bell^ evidently considers that the New Zeahxnd graptolites 

 occur at one horizon, which view, however, is not upheld by 

 Dr. Shakespear (loc. cit., 1908), who points out that there are two 

 zones which are both lithologically and palseontologically distinct. 

 The specimens examined by Dr. Shakespear were obtained from three 

 points extending along a valley for some half-mile. The most 

 southerly of these supplied only the coarse-grained argillite called the 

 a type, while the other two localities supplied both types a and h. 

 In this half-mile or so of valley the dip of the stratum changes 

 considerably, being some 60° at the southern locality to 30° at the 

 northern, the bed becoming alnxost flat as the summit of the syncline 

 is reached. 



The fossiliferous bed is itself but a few inches thick, and is 

 characterised by being more graphitic than the accompanying slate ; 

 and though the bed cannot be traced from one fossiliferous locality to 

 another, yet considering together the change in the strike and the 

 elevation of the places above the level of the creek, the weight of 

 evidence seems to point to the bed being continuous from one place to 

 another, and that only the one bed is present. 



Again, since both types are present in a band but a few inches 

 thick, and if each type is considered representative of a different zone, 

 then between the period of each zone the deposition of the sediments 

 has been almost, if not altogether, suspended. Of this there is a total 

 lack of local evidence. 



It seems highly probable, then, that the horizons have succeeded 

 each other so rapidly that the remains of the two are almost in 

 juxtaposition. 



VII. — The Mechanics of Overthrusts. 

 By T. Mellard Eeade, F.G.S., A.M.I.C.E. 



MK. OSMOND FISHER, in an article in the January number^f 

 this Magazine, entitled " Convection Currents in the Earth's 

 Interior", speaks of my views in a way that may give a false 

 conception of what they are on the subject of overthrusts. He 

 appears (inadvertently, no doubt) to credit me with a disbelief in the 

 thrust-planes occurring in the North- West Highlands. I may say at 

 once that I am one of those who admire the way in which a band of 

 enthusiastic geologists have worked out the structure of this most 

 difficult tract of country, and thoroughly believe in the correctness of 

 their conclusions.- The principal object of my little paper in the 



1 Bull. N.Z. Geol. Survey, 1907, No. 3. 



2 Memoirs of Geological Sui'vey of Great Britain : " The Geological Structure of 

 the North- West Highlands." 



