METAMORPUICS. 167 



In ascending the mountains of Tigrd from Komayli to 

 Senafd, a gradual decrease in the amount of alteration 

 which the beds had undergone is very perceptible. 

 About the lower portion of the pass, near Suru especially, 

 the rocks are much hardened and changed ; but above 

 they are slaty, and in places scarcely crystalline at all. 

 Some beds in the neighbourhood of Senafe are true clay 

 slates ; the original bedding, in one instance at least, 

 being still minutely visible. At first these phenomena 

 appear to indicate a gradual diminution in the amount 

 of alteration corresponding with the- smaller depth of the 

 rocks from the surface ; for it is evident that the deep 

 valleys on the flanks of the Abyssinian mountains have 

 been cut out from the plateau by running water, so as to 

 expose rocks formerly at a depth at least as great as that 

 of the bottom of the valleys below the high ground of 

 the plateau. But it appears most probable that the phe- 

 nomenon is merely local. The high ground near Halai 

 consists of rocks which are more crystalline than near 

 Senafe, and the most crystalline rocks seen, those above 

 referred to as occurring near Keren and Af Abed, are at 

 elevations of from 3,000 to 5,000 feet above the sea. 



The foliation of the metamorphic rocks in Tigr^, so far 

 as they were observed, is remarkably constant in direc- 

 tion ; its strike rarely deviates more than a few degrees 

 from north and south. The dip is more variable, though 

 it is generally nearly vertical on the highlands ; near the 

 base of the hills about Komayli and Hadoda, and again 

 further to the north, about Ain, it is less than 30°. The 

 north and south strike was observed to prevail from the 



