STREAKS AND WEDGES IN CLAYS AND ROCKS. 1G5 



consists of a stratified, light-coloured, yellowish clay 

 such as I have already spoken of as filling the hollows 

 in this county not of recent deposition, and penetrated 

 throughout with long vegetable roots, as one might sup 

 pose the tire-clays of our coal-measures to have originally 

 been. A section of eight or ten feet in depth was visible 

 in some places, and in the mass appeared whiter lines, 

 or vertical plates and w r edges sometimes edged by 

 browner, more ochrey margins parallel, transverse, 

 straight and curved, and running into each other. 

 These had evidently been formed by the filling up, with 

 a whiter or browner material, of the cracks which had 

 naturally formed during the original drying of the clay. 

 The farmer of our stiffest English clay-lands (imdrained 

 Oxford and Weald clays, for example) is familiar with 

 cracks three or four inches wide in long droughts, and 

 to the bottom of which his stick cannot reach. In clays 

 newly deposited, therefore, cracks may be much wider 

 and deeper, especially where a hot sun daily beats upon 

 it. The filling up of such cracks by after floodings, or 

 by natural rains or springs, is the cause of the appear 

 ance I am now describing. 



It is not difficult to understand that white and other 

 coloured streaks, crossing mica-slates and other metamor- 

 phic rocks, may also have been derived, in some cases, 

 from infiltrations like these ; and that, even in metamor- 

 phic granites, the occurrence of streaks and veins of a 

 different colour by no means necessarily implies* that 

 melted matter of a more fluid kind has been injected into 

 the cracks and fissures of cooling or of previously exist 

 ing granite. A clay such as I have described, if sub 

 jected to gradual metamorphic action, might produce in 

 its mass a granite somewhat different from that which 

 would be produced by the vertical plates or wedges with 

 which the cracks had originally been filled up. 



Nov. 1 2. I yesterday attended service in the Epis- 



