THE OLD RED SANDSTONE OF THE CLYDE AREA. 



BY J. G. GOODCHILD, F.G.S. 



THE rocks usually classed under the general name of the Old Eed Sandstone 

 in Scotland consist of four separate formations, three of which are well 

 represented within the basin of the Clyde. These three are widely separated 

 from each other in time, and are linked together chiefly because they 

 collectively occupy a geological position intermediate between that of the 

 Silurian rocks below them and the Carboniferous rocks above. In addition 

 to that they may be said to have had a common origin, seeing that they present 

 various types of rocks formed under continental conditions ; in which respect 

 they differ from the strata of marine origin which occur above and below 

 them. 



The key to their common characteristics is to be found in the fact that 

 they were all deposited during a period of prolonged terrestrial disturbances 

 during which movements of upheaval predominated, and maintained for long 

 periods at a time climatal conditions of exceptional aridity. 



Of the three subdivisions occurring in the basin of the Clyde the lowest 

 has lately been classed as a member of the Silurian rocks. Under the name 

 of the Lower Old Eed Sandstone its geological position was accurately 

 described by Murchison and Geikie. But when the fact came to be known 

 that a newer Old Eed lay unconformably upon it and was in turn uncon- 

 formably succeeded by the true Upper Old Eed, it became evident that some 

 more distinctive name was needed for the lowest strata of the group, and 

 this was more especially the case when palaeontological evidence rendered it 

 clear thatjhe Orcadian rocks were the true Middle Old Eed. I therefore 

 proposed to call these oldest rocks the Lanarkian rocks, with reference to 

 the county in which they are typically represented. When, however, the 

 officers of *the Geological Survey re-examined these rocks, and it was found 

 that they appeared to graduate downward into the Silurian rocks, and that 

 they were linked on to them further by containing an ichthyic fauna having 

 the same facies as that found in the associated strata of Silurian age, the 

 name Downtonian was proposed instead, and is now coming into use. 



The Downtonian rocks consist of alternating beds of sandstone and mud- 

 stone, characterised by reddish and greenish-grey colours, with which are 

 associated one or more bands of fine conglomerate, whose predominant con- 

 stituents are small pebbles of quartzite, with little or no intermixture of 

 pebbles of greywacke. These strata are interstratified with others of the 

 petrographical types prevalent in the underlying Ludlow rocks. No traces 

 of any volcanic rocks have yet been found in them. 



The entire aspect of the group is exactly what might be expected in the 

 case of strata formed under transitional conditions when the area in which 

 they occur was gradually passing from an area of subsidence to one of 



