524 Reports & Proceedings — British Association. 



most characteristic of S. Corals attain their greatest importance 

 in D. 



Crinoidal Limestones: Crinoids are abundant in K x and K 2 , and 

 are the gi-eatest limestone builders throughout the whole of the 

 Z beds. 



Brachiopod Limestones are met with throughout nearly the whole 

 section. Spirifer, Orthotetes, and Chonetes being the most abundant 

 Tournaisian genera, Seminula, Productus, and Chonetes the commonest 

 Yisean . 



Ostracods are very plentiful wherever the rocks are shaly or of 

 the calcite-niudstone type, viz.: throughout K, at the top of Ca, and 

 in the lower part of Sj. 



Oolites occur at the following levels : (a) in the upper part of G u 

 (b) in the middle of S 2 , (<?) throughout D. 



Siliceous Pocks. 

 Grits are met with only in the D beds. Chert bands occur («) 

 near the middle of Z u (b) in S 2 below the oolite, (c) in S 2 between 

 the oolite and the " Concretionary Beds ". 



■ Argillaceous Hocks. 



Thick shales are met with (a) throughout K 1} (b) in upper Ca and 

 lower S 1( (c) in upper D T and upper D 2 . 



Changes which have affected certain of the rocks. Penecontemporaneous 

 hrecciation (desiccation breccias) are characteristic of all the shallow 

 water (lagoon-phase, Dixon) rocks of C 2 and S. 



Dolomitization proves to be considerably more widespread in the 

 Avon rocks than had been previously supposed. The matrix of the 

 Petit Granit of Zi, Z 2 , and <y is almost everywhere dolomitized. 

 The almost complete dolomitization of Ci and the upper part of C2 

 has long been familiar. There has been considerable dolomitization 

 in the calcite-mudstones of Si and lower Sa. All the chief dolomites 

 are to be classed as contemporaneous according to the classification 

 of Mr. L. M. Parsons. 1 



4. The Pre- Cambrian oe Central Canada. By "Willet G. Miller. 



Ten years ago, at the Winnipeg meeting of the British Associa- 

 tion, the author presented a paper dealing with the age relations of 

 the pre-Cambrian rocks of Canada. Since then much field work has 

 been done in connexion with these rocks, not only in the province of 

 Ontario, but to the eastward in Quebec and, to a lesser extent, to the 

 westward in Manitoba and Saskatchewan. There has been great 

 mining activity in the pre-Cambrian areas of Ontario, which has 

 afforded special facilities for study to the geological staff of the 

 Ontario Bureau of Mines. From time to time papers and reports 

 have been published as our knowledge has increased, and the age 

 classification has been revised. 2 The following classification is now 

 employed by the Ontario Bureau 3 : — 



1 Geol. Mag., Dec. VI, Vol. V, p. 246, 1918. 



2 (hit. Bur. Mines, vol. xix, pt. ii ; vol. xxii, pt. ii. 



3 Journal of Geology, vol. xxiii, No. 7. 



