EFFACEMENT OF OBTGINAL CHARACTEEISTICS 249 



In eases where trappean dikes have cut through friable sand- 

 stones, they have in some instances so indurated these rocks 

 along either contact as to cause them to be more durable than 

 the original rock or than even the trappean rock itself. There 

 may thus arise long parallel ridges of indurated sandstone sepa- 

 rated by an intervening depression due to the weathering out 

 of the dike material. 



In regions where climatic conditions or the nature of the rock 

 are more favorable to mechanical disintegration than chemical 

 decomposition, contours may be ragged in the extreme. Entire 

 crests may be but successions of jagged peaks and intervening 

 narrow valleys which are gradually becoming choked up by the 

 debris fallen from the cliffs above. 



(10) Effacement of Original Characteristics through Weath- 

 ering. — In cases of extreme decomposition, in place, the residual 

 products ma3^ so slightly resemble the parent rock as to give rise 

 to very conflicting opinions concerning their origin. This was 

 for a long time the case with the laterite of India, already 

 described, and the terra rossa of Europe. 



Dana describes^ an interesting case of basaltic decomposition 

 which, on account of the peculiar nature of the residual product, 

 is worthy of mention here. He writes : ^ ' The process of decom- 

 position is finely exhibited on the second cliff north of Kiama 

 (Australia) towards the north end. At first sight, a distinct 

 argillaceous deposit was supposed to overlie the columnar basalt ; 

 for it was twenty feet thick, and of a whitish color, resembling 

 a soft crumbling marl, thus wholly unlike the basalt, and the 

 common results of basaltic decomposition. Still it had pro- 

 ceeded from the alteration of a regular columnar variety, having 

 a dull grayish blue color. The original rock is exceedingly 

 compact, showing no trace of crystallization, excepting an oc- 

 casional minute crystal of feldspar ; and within the reach of the 

 swell, it was still compact and solid. 



'^The rock has a concentric structure, and to this it owes in 

 part its rapid decomposition. The alteration commences be- 

 tween the concentric layers, rendering them apparent, although 

 not so before. At first a thin ochreous line appears, arising 

 from iron; either magnetic iron disseminated in the rock, or 

 from that of the constituent mineral augite. This ochreous 

 color afterwards mostly disappears, and the concentric coats 



^Beports Wilkes's Exploring Expedition, Geology. 



