364 G. W. Tyrrell — Intrusions of KilsytJi-Croy District. 



anything from 50 feet to 500 j-ards wide. The latter dyke causes the 

 great Tumatumari Falls on the Potaro River. ^ In South Victoria 

 Land the rock occurs as great sills in the Beacon Sandstone, and at 

 one point, at least, a large dyke or pipe of dolerite, 50 yards wide, was 

 found cutting vertically through the sandstone beneath a sill.- 



TABLE III. 



I. Diabase, Mt. Hol}'oke, Massachusetts.^ 



II. Diabase, dyke, Potaro River, British Guiana.* 



III. Dolerite (with Micropegmatite), Knob Head, South "Victoria Land.^ 



IV. " Augite-Diorite with Micropegmatite," Seven Pagodas, Chingelput, India.* 



In Scotland the great dykes occur as an after-phase of great extru- 

 sions of basic lavas. In India the augite-diorite dykes are believed 

 to be associated with the basic Cuddapah lavas. Lawson suggests 

 a relation between the great dykes of the Rainy Lake region and the 

 Animikee and Keweenawan traps of that district. In Skye and other 

 British centres of Tertiary volcanic activity, great extrusions of plateau 

 basalts are followed by the intrusion of gabbro and granophyre in 

 intimate relations, rocks which, if mixed a little more thoroughly, 

 would be identical with granophyric diabase. 



1 Harrison & Perkins, Rep. on Geol. of Essequiho and Potaro Rivers, George- 

 town. 1900, p. 35. Also Harrison, Geol. of the Ooldfelds of British Guiana, 1908. 



- Ferrar, Nat. Antarctic Exp., 1907, vol. i, " Geology," p. 50. 



2 Hawes, Amer. Journ. Science (3), 1875, vol. ix, p." 186. 



* Harrison & Perkins, Rep. on Geol. of Esseqtdbo and Potaro Rivers, 1900, p. 64. 



5 Prior, Nat. Antarctic Exp., 1907, vol. i, p. 137. 



6 Holland, Q.J.G.S., 1897, vol. liii, p. 409. 



