470 Notices of Memoirs — Br. Woolacott — Magnesian L. Breccias. 



the yellow sands, marl slate, and Magnesian Limestone, but the effect 

 of the thrust on the first two cannot be observed. The limestone, 

 although much disturbed, has a general low dip to the south. It 

 consists of a series of rocks of different flexibility, rigidity, brittleness, 

 and compressive strength, and the uniqueness of the section is due to 

 the action of a thrust from the north, which has caused some of the 

 beds to move laterally upon the others, with an associated production 

 of folding, minor-thrusting, and Assuring, and a consequent develop- 

 ment of dynamic brecciation. 



In Frenchman's Bay the lower limestone (40 feet thick) is 

 a brownish-yellow, regularly bedded rock of relatively high com- 

 pressive strength and rigidity. Its lower layers are gently folded, 

 but its top layers are considerably disturbed, being fractured, tilted 

 up, and laterally displaced. Resting on this is about 50 feet of 

 brecciated limestone, consisting almost entirely of a cemented mass 

 of broken fragment, which have here and there been dissolved out 

 from the cementing matrix, the rock becoming cellular. In places 

 the bedding, unfolded but fractured, is still preserved. It was 

 originally a finely laminated granular limestone, and is of low rigidity 

 and compressive strength. Experiments performed by Dr. Morrow 

 give the following results : — Compressive strengths (tons per square 

 inch to break rock), lower limestone, 5-7 tons ; brecciated bed (lower 

 middle), 1 ton. 



The junction of the brecciated beds with the lower limestone is 

 a thrust-plane, which does not always coincide with the line of 

 demarcation between the two strata ; and the disturbance of the upper 

 layers of the latter rock, together with the smashing up of the former, 

 is due to the thrust movement. 



The brecciated beds occupy the top of the cliff for over a mile. 

 Their upper surface, which is seen at the north end of Marsden Bay, 

 is very irregular and hummocky, the breccia having been forced up 

 into the base of the beds above. These consist of about 200 feet of 

 rock differing much in flexibility and compressive strenjith (specimens 

 tested vary from 1 ton to 37 tons per square inch). They have been 

 thrust against a 'Horst," and consequentlj' folding, thrusting, and 

 dynamic brecciation have taken place. The flexible beds have been 

 deformed without being much broken, while a harder, more brittle, 

 wedge-shaped limestone has been highly brecciated. The latter has 

 also had a coarse cleavage structure impressed on it, and part of it has 

 been torn off and thrust into the beds above. Breccia gashes and 

 vertical fissures filled with breccia, which has fallen into them, occur 

 on both sides of this folded and broken area. The amount of lateral 

 displacement at this point has probably been about 100 yards, and the 

 experiments indicate that the magnitude of the thrust was about 

 300 tons per square foot. 



1 'Horst,' see Suess, " Antlitz der Erde," 1st ed. (1885), vol. i, p. 167, and also 

 Maria Ogilvie, Quart. Joum. Geol. Soc, 1893, vol. slix, p. 77, for explanation 

 of term. 



