EX P13, UROGINA IP IsUUCANL, “Sis 1Clal Ole AaGluNva, Ono) 
METHANA. 
PART II. PETROGRAPHICAL DESCRIPTION.* 
Classification.—W hile limestones and other non-eruptive rocks 
occur in the region at present under description, yet they offer 
no special features of interest and will not be touched upon fur- 
ther. What will chiefly engage our attention are the eruptive 
rocks proper which make up by far the greater part of A‘gina 
and Methana, with the eruptives from the neighboring and closely 
related localities of Poros and Kolautziki. Some space will also 
be devoted to the segregations found in the lavas and to the 
tuffs. 
The eruptive rocks may all be referred to the two well defined 
groups of andesites and dacites, but a few words must be 
devoted to the classification here adopted before we begin their 
description. 
The distinction between the two main groups, as defined by 
Zirkel,? does not depend solely on the presence or absence of 
quartz but on the superabundant amount of SiO, whether it has 
crystallized out or not, exactly as is the case with the trachytes 
and rhyolites. In the case of the rocks before us there is, as 
usual, some difficulty in drawing a fast line between the two; 
but, as a basis for the distinction | have used the percentage of 
silica, those rocks having over 62 per cent: being classed as 
dacites, those with less as andesites. As it was impracticable to 
have analyses made of all the specimens I have had to judge in 
some cases by the presence or absence of quartz grains, or by 
analogy and association, so that, though we may be left in doubt 
in a few cases as to where to place a given specimen, yet on the 
whole the discrimination is made with comparative ease, and the 
‘Continued from Vol. II., p. 813. 
? ZIRKEL, Lehrb. v. Petrog. 2d ed., 1894, p. 569. 
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