Ch. XI.] CHAEACTERISTICS OF GLACIAL DRIFT. 137 



the deposit the name of till lias long been applied in Scotland. It 

 generally contains a mixture of angular and rounded fragments of 

 rock, some of large size, having occasionally one or more of their 

 sides flattened and smoothed, or even highly polished. The smoothed 

 surfaces usually exhibit many scratches parallel to each other, one set 

 of which often crosses an older set. The till is almost everywhere 

 wholly devoid of organic remains, except those washed into it from 

 older formations, though in some places it contains marine shells of 

 arctic species, many of them in a fragmentary state. The bulk of the 

 till has usually been derived from the grinding down into mud of 

 rocks in the immediate neighborhood, so that it is red in a region of 

 Red Sandstone, as in Strathmore in Forfarshire ; gray or black in a 

 district of coal and coal-shale, as around Edinburgh. ; and white in a 

 chalk country, as in parts of Norfolk and Denmark. The stony frag- 

 ments dispersed irregularly through the till usually belong, especially 

 in mountainous countries, to rocks found in some parts of the same 

 hydrographical basin ; but there are regions where the whole of the 

 boulder clay has come from a distance, and huge blocks, or " erra- 

 tics," as they have been called, many feet in diameter, have not unfre- 

 CHiently travelled hundreds of miles from their point of departure, or 

 from the parent rocks from which they have evidently been detached. 

 These are commonly angular, and have often one or more of their 

 sides polished and furrowed. 



The fundamental rock on which the boulder formation reposes, if it 

 consists of granite, gneiss, marble, or other hard stone, capable of per- 

 manently retaining any superficial markings which may have been im- 

 printed upon it, is usually smoothed or polished, like the erratics above 

 described ; and exhibits parallel strias and furrows having a determinate 

 direction. This direction, both in Europe and North America, agrees 

 generally in a marked manner with the course taken by the erratic 

 blocks in the same district. 



The boulder clay, when it was first studied, seemed in many of its 

 characters so singular and anomalous, that geologists despaired of ever 

 being able to interpret the phenomena by reference to causes now in 

 diurnal action. In those exceptional cases, where marine shells of 

 the same date as the boulder clay were found, nearly all of them were 

 recognised as living species — a fact conspiring with the superficial 

 position of the drift to indicate a comparatively modern origin. The 

 recentness of the date caused the enigma to appear only the more 

 perplexing, and strengthened the belief that the phenomena were the 

 results of forces distinct both in kind and energy from those now op- 

 erating in the ordinary course of nature. Notions of this kind were 

 calculated to retard the progress of science, by diverting attention 

 from such every-day operations as were capable of producing analogous 

 effects. 



The term " diluvium" was for a time the most popular name of the 

 boulder formation, because it was referred by many to the deluge of 



