180 
The American Geologist. 
March, 1396 
author permit of no doubt of the igneous origin of the halleflinta and 
halleflinta-gneiss of Sjogelo. 
They separate into the following groups : 
Hypidiomorphic granular rocks: 
Granite, aplite, gabbro. 
Porphyritic rocks: 
Occurring as massive rocks, 
Granite porphyry and microgranite. 
Granophyre and an augite-bearing-porphyritic granopkyre. 
■ Occurring as dyke-rocks, 
Granite-porphyry, diorite-porphyrite, uralite-diabase-porphyrite. 
Effusive rocks. 
Eorhyolite, eodacite, pyroclastics, diorite, augite-porphyrite. 
The granites are fine grain, aplitic and frequently show a gneiss-like 
alternation of light and dark bands, which the author is inclined to 
consider an original fluidal structure. The chief interest of the granites 
lies in the fact that they merge into halleflintas through this gneissoid 
banding, or fluidal structure, and an increasingly finer crystallization. 
The gabbros, also, stand in the same close relation to the basic mem¬ 
bers of the halleflinta formation. 
The porphyritic rocks constitute another transition between the 
granites and the halleflinta proper. Granite porphyries, formerly denom¬ 
inated halleflinta or halleflinta-gneiss, possess a microgranitic ground- 
mass in which are imbedded phenocrysts of orthoclase, plagioclase, 
and biotite. The microgranites form the chief mass of the halleflinta 
and are both porphyritic and non-porphyritic. They are characterized 
by a finely crystalline groundmass closely related in structures to that 
of the eorhyolites. Sericite is often abundantly developed. There are 
typical granophyres which, like the quartz-porphyries of the South 
mountain, pass into sericite schists. 
The porphyritic dyke-rocks occur as composite dykes some 40 meters 
wide and consist of an acid porphyritic centre with a selvage of diabase- 
porphyrite. Orthoclase is essential to all of the acid types. Quartz is 
often missing without the composition of the rock becoming more basic. 
Hornblende is rare. The groundmass is a finely crystalline mosaic of 
quartz, orthoclase, plagioclase and microcline. The effect of pressure 
is seen in cracks and undulatory extinction. For petrographical des¬ 
criptions of the basic dyke-rocks (diorite-porphyrite and uralite-diabase- 
porphyrite) the author refers to the papers of Eichstadt and Holst. 
The ancient acid effusives occur in the greatest perfection in the Sjo¬ 
gelo district. They constitute almost half of the entire district, and 
while they closely resemble the English, Belgian and American ancient 
rhyolites, nowhere else have they shown a transition into granites. 
These ; ‘eorhyolites” are characterized in the hand specimen by a dense 
grayish black groundmass, which may or may not be porphyritic. When 
phenocrysts are present, they are exceedingly numerous and especially 
conspicuous upon the weathered surface. They are plagioclase, ortho¬ 
clase, biotite or quartz. The accessory constituents are magnetite, il- 
menite, titanite, apatite and zircon. The secondary constituents are 
caleite, epidote, chlorite, fluospar. rutile and prehnite. 
