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, picturesque in their desolation, which 
Von Buch has termed the " Felsen-Meere" or " seas of rocks." 
Between these gigantic fragments 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 
quai-tz-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 structiire ; 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 
gi-anite-masses weather into curved lamince 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 comers 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 will quote Prof. W. Macgillivray's 
description of a part of Scotland : — 
