182 
The American Geologist. 
March, 1896- 
ered surface, the centres of the spherulites are vesicular or filled some¬ 
times with calcite.- This was not infrequently the case with the South 
Mountain aporhyolites.and has led the reviewer to conclude that sphero- 
litic centres are points of easy solution and redeposition. 
The chemical composition as well as the structures ally these ancient 
effusives to their modern equivalents, while the preponderance of soda 
over potash relate them to the keratophyres and. dacites. That a con¬ 
siderable manganese percentage is always present is noticeable. 
Basic Volcanics : The greenstones or dioritic rocks found on the 
west side of lake Kalsjon possess a diabase-porphyritic structure, which 
is nearly obliterated by alteration. The chief constituents are secondary 
epidote and chlorite. The augite-porphyrites still show a typical struc¬ 
ture. The original constituents, however, are almost completely re¬ 
placed by epidote, chlorite and biotite. 
The Smaland halleflintas are the oldest known of the Swedish volcanic 
rocks. Sederholm separates the pre-Cambrian formations of Sweden 
into three divisions : The Katarchsean complex, the Bottnian system 
and the Karelian system (Algonkian.) The halleflintas of Smaland are 
placed in the Katarchsean complex, while the halleflintas of Elfdalen 
are Karelian, and Sederholm’s uralite-porphyrites are Bottnian. 
In dealing with the question of nomenclature the author reaches the 
following conclusions: These halleflintas are not the felsophyres or 
vitrophyres of Rosenbusch, as they contain neither glass nor true micro- 
felsite. They are identical with the ancient rhyolites of Allport, Rut- 
leo, De La Vallee Poussin and Williams. They are, however, in every 
case so different from typical rhyolites that a cursory examination 
serves to separate them. These constant differences should be recog¬ 
nized in a name. Felsite has been used by English petrographers and 
by Stolpe among Swedish petrographers for rocks of this class. This 
term is to be discarded however on the ground that it already has a 
definite meaning, viz., a non-porphvritic quartz-porphyry. In view of 
these facts the author proposes the name eorhyolite for the ancient de- 
vitrified rhyolites, and suggests that in an analagous way the terms 
eoandesite , eobasalt, etc., should be formed: also that palaeo- (or meso-) 
and neo- should be prefixed to the effusives of those respective ages. 
The author recognizes the fact that there is no definite petrograph- 
ical distinction between pre-Tertiary and post-Tertiary volcanics, but a 
difference in habit is well established. This difference in habit is shown 
for instance in the frequent occurrence of an adiognostic or cryptocry¬ 
stalline grounclmass containing sericite and porphyritic constituents, 
with indistinctly idiomorphic boundaries (through secondary growths?), 
in the abundance of secondary constituents and in the appearance of the 
spherulites, etc. As to the cause of these differences between ancient 
and recent volcanics there can be but two views: (1) An orig¬ 
inal difference in the conditions of eruption; the cause of this 
original difference is partially unknown, and is partially to be explained 
in connection with the cooling of the earth’s crust. This idea justifies 
the establishment of a class of rocks older than and unlike the palmo- 
