260 T. Mellard Reade—On the Lower Trias. 



seen his way to the adoption of what is substantially the explanation 

 offered by myself. In a paper read before the Geological Society of 

 America 1 (dated March, 1890), he says, in speaking of the Significance 

 of the Relationship [of the gneiss-series and the schist-series] : 

 — " Bearing in mind the essential distinctions which exist between 

 the rock-formations of the Ontarian 2 and Laurentian systems, both 

 as to their lithological character and their mode of occurrence, and 

 remembering also their relative geographical distribution, 3 the rela- 

 tionship which obtains between the two systems leads clearly and 

 unavoidably to this conclusion, viz. that the formations of the 

 Ontarian system at one time rested, as a volume of hard rocks, upon 

 a magma which subsequently crystallized as the Laurentian granite- 

 gneiss ; so that the present line of demarcation between the two 

 systems must be regarded as representing the trace of what was 

 once a plane of contact between the thin crust and the magma upon 

 which it floated. 



" This conclusion affords a conception of the Archasan which is 

 ideal in its simplicity, and which gives us the key to the ravelling 

 of the mystery in which the subject has been involved. The fact 

 that the crust, which constitutes what we now call the Ontarian, 

 was crumpled while it floated on the magma ; the fact that its lower 

 portions were shattered by disturbance, so that the magma penetrated 

 the fissures and inclosed detached fragments ; the fact that there 

 were currents in the magma which arranged the inclusions in 

 streams and so produced the foliation of the gneiss ; the fact of 

 contact-metamorphism ; — all these are incidental and concomitant 

 circumstances of the great essential condition of a crust resting on 

 a magma." 



[See a most excellent Eeview of Dr. Andrew C. Lawson's 

 Memoir on the Geology of the Rainy Lake Eegion (being Part F, 

 of the Annual Eeport of the Geological and Natural History Survey 

 of Canada for 1887), in the Geological Magazine for January, 

 1890, Decade III. Vol. VII. pp. 36-39.— Edit. Geol. Mag.] 



V. — Physiography of the Lower Trias. 

 By T. Mellard Eeade, C.E., F.G.S. 



EBEE discussion is a valuable aid to research when entered upon 

 in a proper spirit. I am glad to see the contribution of Mr. 

 Jukes-Browne to this subject, together with the letter from Prof. 

 Bonney, in the May Number of the Geological Magazine. 



Having put forward a theory of the origin of the Lower Trias, 

 I cannot object to its being tested in every way ; so in this spirit I 

 will proceed to answer as best I can the criticisms which it has 

 called forth. 



It appears to me that Mr. Jukes-Browne has scarcely grasped the 



1 See " Bulletin of the Geol. Soc. of Am." vol. i. pp. 175-194. 



2 An inclusive term now proposed for the Upper Archtean [= Coutchiking and 

 Keewatin series.] 



3 Lawson estimates the Upper Archaean as easily 9 miles in thickness ; but they 

 are small in extent compared with the gneiss series. 



