318 Horner's Geological Address. 



of rocks crossed, but always indicating a southerly direction 

 of the drift, the Devonian detritus never being found in the 

 Silurian zone, nor the carboniferous in the Devonian zone. 



Mr Forchhammer describes the boulder formation of Den- 

 mark as being of different ages. The oldest which affords 

 any distinct evidence to mark its age, consists of a congeries 

 of clays, marls, and sands, which have been traced to a depth 

 of several hundred feet, and contain boulders throughout the 

 entire mass, extending to the deepest part of the series. The 

 boulders, sometimes several hundred cubic feet in size, are of 

 granite, gneiss, porphyry, greenstone, and quartz rock, and 

 also of transition (Silurian) sedimentary rocks ; none of these 

 occurring nearer than Norway and Sweden. Besides these 

 travelled blocks, there are many parts of the formation com- 

 posed of chalk, identical with rocks upon or near to which the 

 boulder formation occurs. In the duchy of Schleswig, this 

 boulder formation alternates with beds of brown coal, a deposite 

 which exends over the greater part of Denmark, and which, 

 besides brown coal, consists of clays, limestones, and sand- 

 stones, containing fossils, that, in the opinion of Mr Forch- 

 hammer, mark it to be identical with the sub-Appenine group. 

 The causes which produced this boulder formation, in part at 

 least, were therefore in operation as early as the Miocene 

 tertiary period (if, as some maintain, the sub-Appenines are 

 of that age), during which the sea, overspread at its bottom 

 by this detritus, was inhabited by Mediterranean species. 

 There is clear evidence in the works of the authors I have 

 quoted, of the operation of the same causes long after the 

 northern seas were inhabited by existing species ; and 

 throughout the whole of this period, how long we have no 

 means of determining, all the land in Northern Europe over- 

 spread by the boulder formation must have been under the 

 sea. Thus the authors of the " Geology of Russia" describe 

 the deposite of recent shells in the valley of the Dwina, 150 

 miles inland from Archangel, as covered by sand and gravel, 

 which, they say, they would have great difficulty in separating 

 from the superficial northern drift ; and they add, that " a 

 recent excursion through Sweden has convinced them that in 



