Correspondence. 341 
parative ages of the granites and the Bothnian schists. In this question 
is concerned the age of a large area of granite to the south. 
Sederholm considers that the contact between the porphyritic granite 
and the schists is a mechanical one and that this granite is older than 
the schist, which includes some fragments of it. While the contact of 
the schist and the nonporphyritic granite he considers intrusive and 
this granite younger than the schist. 
To many of the party the contact in both cases appeared to be of an 
intrusive character, and the non-porphj'ritic granite to be afaciesof 
the porphyritic granite and not to be distinguished as a separate form- 
ation. With this interpretation both granites are younger than the 
schist. 
Two following days were devoted to glacial geology and to banqueting. 
The asar of Finland are striking physiographic features. The as of 
Tammersfors, the as of the church of Kangsala and the Kaiser as were 
visited by the party. These long, narrow ridges constitute the 
"mountains" of Finland and from their summits the views are always 
beautiful. They are interpreted as formed by the retreating delta of 
glacial streams. Remarkably fine examples of rock scouring and 
striations occur abundantly in Finland and- on the island of Hogland. 
The last day of the excursion was spent upon the island of Hogland. 
This rocky island, in the middle of the gulf of Finland, is about 11 km. 
long and from l^fj to .3 km. wide. 
The interest of the island lies in the fact that it presents in epitome 
the larger number of the various formations which constitute the main- 
land of Finland. 
The highest land (158 m. high) and the greater part of the island is 
composed of quartz-porphyry, corresponding in age to the Rapakivi 
gi-anite of the mainland. (Jatnian or Algonkian). The other forma- 
tions of the island are all older and are as follows, in order of age from 
the oldest upward. (1) crystalline schists (Archaean). These schists 
are impregnated with granitic injections, [f) an uralitized gabbro, 
showing evidences of dynamic action. The intrusive granite of the 
crystalline schists, is also intrusive in the gabbro but not in the 
overlying formations. (3) A quartzite and eurite, which are free from 
granite intrusions, are subsequent to these intrusions and are thus sep- 
arated by a discordance from the gabbros. They, unlike the more 
ancient schists, still show a clastic structure, associated, however, 
with metamorphic minerals. 
These quartzites are, in turn, overlaid discordantly by (4) a basal 
conglomerate formed of pebbles of the quartzite. A finer grained 
quartzite is contemporaneous with this conglomerate. (5) In contact 
with this younger quartzite, which is then altered to a hornstone, and 
with the conglomerate, which is sometimes caught up into the over- 
lying rock, is an effusive of great petrographical interest and beauty. 
It has been called a labrador-porphyrite. It is a dark green fine- 
grained rock with slender lath-shaped phenocrysts of a light colored 
twinned feldspar. The phenocrysts exhibit a conspicuously trachytic 
