CRYSTALLINE BASEMENT 41 
stage of beginning consolidation in progress, when the pegmatitic 
pe cut through the zone in question. 
"> These dikes, broad and light coloured, very markedly visible in the 
| rock ground, are highly differentiated. The border zones are formed 
: of a red feldspar and the midst of the dike is filled with milkwhite 
quartz. Frequently there is also a smaller exterior border outside 
h the feldspar mass consisting of an evengrained aplite. The limits 
between the feldspar and the quartz masses are quite distinct. The 
i thickness of the dikes varies 
; much and attains several times 
some meters. It may be noted, 
that the differentiation appears 
with the same distinction in 
dikes of various breadth. 
The quartz mass contains 
often well developed crystals 
of a black tourmaline (schorl). 
The feldspar is a red ortho- 
clase and forms, when isola- 
ted, great individuals (see 
Fig. 6. Great individuals of red potash feldspar R 3 é 
ying in a milkwhite quartz mass. The cleavage lig. 6), never mingled with 
ee nel Guar, |The habit of these 
Om Wilke: Ural individuals is idiomorphic in 
some degree, showing partly the pinacoids well developed. Pneu- 
‘matolytic minerals are scarcely existent, except the above named 
tourmaline. Beryl was once found in a pegmatite and was of 
greater dimensions with the prismatic faces striated (Cerro La Vatea). 
The flat lying position of the dikes is difficult to explain with 
regard to the general steep inclination of the schists intruded with 
| granite in this zone. The pegmatitic eruptive material has appa- 
rently followed independent fissures, opened subsequently to the 
consolidation of the mingled-rock-zone. 
The origin of pegmatites has been regarded, as known, partly as 
