308 



THE GEOLOGIST. 



this condition, aided by their partial destruction by the atmosphere, 

 has led to the formation of those rugged fields of loose irregular 

 angular blocks and masses, pictui^esque in their desolation, Avhich 

 Von Buch has termed the " Felsen-Meere" or " seas of rocks." 

 Between these gigantic fragTiients and the sand of the " granit 

 pourri" is a wide difference as to appearance ; but the peculiar joint- 

 ings of the rock and the rottenness of the felspar lead alike to these 

 two conditions of granite rock. The "granit pourri," consisting of 

 quartz-grains and half destroyed crystals of felspar, with or without 

 minute flakes of mica, when cemented with silex, which has been 

 dissolved by water and re-arranged among the sandy materials, 

 becomes "arkose," and frequently resembles real granite so closely 

 that only a practised eye can recognize the difference. 



Igneous rocks, such as basalt and some trappean rocks, frequently 

 harden into nodular masses having a concentric structure ; and when 

 these concretions have pressed closely upon each other in the process 

 of cooling, a prismatic or columnar structure has been formed in the 

 rock. Sometimes this concretionary structure is only visible when 

 the mass of trap is decomposing, or when the prisms are broken. 

 So also in granite and some allied forms of rock, we have occasionally 

 a nodular structure. This is seen, in the same way, in the so-called 

 " Corsican granite" or " Neapolonite" ; and small globular lumps 

 are not uncommon in the granite of Dartmoor. But some great 

 granite-masses weather into curved lamina of rock, showing indica- 

 tions of concretionary form on the large scale (see fig. 5). Indeed 

 the rounding off of the angles of the great horizontal slabs forming 

 Haytor (fig. 3), and the " Cheesewring" (fig. 2) seem to point to 

 the fact that nearly every mass or block limited by the intersecting- 

 joints in granite may be regarded as having a nucleus of its own, or 

 a central point within it from whence the crystallization began on 

 cooling, and that the corners of those blocks would the most readily 

 exfoliate on account of their being most independent of the harden- 

 ing influence of such concentric crystallization. 



By way of comparison with and in illustration of the granite of 

 Dartmoor and Cornwall, some of the features of which are shown in 

 the accompanying figures, I vnU qnote Prof. W. Macgillivray's 

 description of a part of Scotland : — 



