J. Croll. — Boulder-clay of Caithness. 277 



level. This table-land would terminate in the deep waters of the 

 Atlantic by a perpendicular wall of ice, extending probably from the 

 west of Ireland away in the direction of Iceland. From this 

 barrier icebergs would be continually breaking off, rivalling in 

 magnitude those which are now to be met with in the Antarctic 

 Seas. 



The Great Extension of the Loess Accounted For. 



An effect which would result from the blocking up of the North 

 Sea with land-ice, would be that the waters of the Ehine, Elbe, and 

 Thames would have to find an outlet into the Atlantic through the 

 English Channel. Mr. Geikie has suggested to me that if the 

 Straits of Dover were not then open — quite a possible thing — or 

 were they blocked up with land-ice, say by the great Baltic glacier 

 crossing over from Denmark, the consequence would be that the 

 waters of the Ehine and Elbe would be dammed back, and would 

 inundate all the low-lying tracts of country to the south. This he 

 thinks might account for the extraordinary extension of the Loess in 

 the basin of the Ehine, and in Belgium and the north of France. 



Postscript. — Since the foregoing was written I have met with a 

 striking confirmation of the opinion expressed, " That the great mass 

 of ice entering the Firths of Forth and Tay could not have been less 

 than from 1,000 to 2,000 feet in thickness." One day lately, along 

 with Mr. James Bennie, I ascended AUermuir, one of the hills 

 forming the northern termination of the Pentland range, and was 

 not a little surprised to find its summit ice-worn and striated. The 

 top of the hill is composed of a compact porphyritic felstone, which 

 is very much broken up ; but, wherever any remains of the original 

 surface could be seen, it was found to be polished and striated in a 

 most decided manner. The stride, we found, were all in one uniform 

 direction, nearly east and west. On minutely examining the ice- 

 markings we had no difficulty whatever in determining that the ice 

 which effected them came from the west and not from the east. The 

 direction of the striae clearly shows that these markings must have 

 been made at the time when, as is well known, the entire Midland 

 valley was filled with ice, coming from the north-west Highlands. 

 On the summit of the hill we also found patches of Boulder-clay in 

 hollow basins of the rock. At one spot it was upwards of a foot 

 in deptb, and resting on the ice-polished surface. The clay was 

 somewhat loose and sandy, which might be expected of a layer so 

 thin, exposed to rain, frost, and snow during the long course of ages 

 which must have elapsed since it was deposited there. Out of 100 

 pebbles collected from the clay, just as they turned up, every one of 

 them, with the exception of three or four, composed of hard quartz, 

 presented flattened and ice-worn surfaces ; and forty-four were dis- 

 tinctly striated; in short., every stone, which was capable of receiv- 

 ing scratches and of retaining them, was striated. A number of 

 these stones must have come from the Highlands to the north-west. 



The height of AUermuir is 1617 feet, and, from its position, it is 

 impossible that the ice could have gone over its summit, unless the 

 entire Midland valley, at this place, had been filled with ice to the 



