56 



DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



probably also of still older Palaeozoic rocks, and beneath all the thick Archaean platform. 

 Tabulating these successive geological formations, and taking only the ascertained 

 thickness of each in the district, we find that they give the results shown in the 

 subjoined table.* 



Feet. 



Strata Cut by the Cleveland Dyke. 



Cornbrash — 



Lower Oolite and Upper Lias, as proved by bore-hole on Gerrick Moor, . . . 950 



Middle and Lower Lias, ascertained from measurement of cliff-sections and from mining 



operations to be more than ........ 850 



New Red Sandstone and Marl, found by boring close to the Tees to exceed 

 Magnesian Limestone, at least 

 Coal-Measures, possibly absent 



Millstone Grit, not less than 

 Carboniferous Limestone series, at least 

 Silurian rocks, probably not less than 



1,600 



500 







500 



3,000 



10,000 



17,400 



There is thus evidence that this dyke has risen through probably more than three 

 miles of stratified rocks. How much deeper still lay the original reservoir of molten 

 material that supplied the dyke we have at present no means of computing. 



§ 12. Branches and Veins. 



It might have been anticipated that the uprise of such abundant masses of 

 molten rock in so many long and wide fissures would generally be attended with 

 the intrusion of the same material into lateral rents and irregular openings, so that 

 each dyke would have a kind of fringe of offshoots or processes striking from it 

 into the surrounding ground. It might have been expected also that dykes would often 

 branch, and that the arms would come together again and enclose portions of the rocks 

 through which they rise. But in reality such excrescences and bifurcations are of 

 comparatively rare occurrence. As a rule, each dyke is a mere wall of igneous rock, with 

 little more projection or ramification than may be seen in a stone field-fence. Among 

 the short, narrow, and irregular dykes of the gregarious type branchings are occasionally 

 seen, and in some districts are extraordinarily abundant. But among the great single 

 dykes such irregularities are far less common than might have been looked for. A 

 few characteristic examples from each type of dyke may here be given. 



The Cleveland dyke, which in many respects is typical of the great solitary dykes of 

 the country, has been traced for many miles without the appearance of a single offshoot 

 of any kind. Yet here and there along its course it departs from its usual regularity. 

 As it crosses the Carboniferous tracts of Durham and Cumberland, there appear near its 

 course lateral masses of eruptive rock, most of which doubtless belong to the much older 

 " Whin Sill." But there is at least one locality, at Bolam near Cockfield, in the county 



* Drawn up for nie by Mr Barrow. 



