CHAP. .'CXVI 
MATERIALS ERUPTED BY THE PUYS 
417 
of the plateau lavas to those of the pays at once remarks the prevalent 
more basic character of the latter. The great majority of them are basalts, 
generally oli\’ine-bearing, in the various types embraced in the table on the 
following page. Tlie olivine-free dolerites are generally found as intrusive 
bosses, sills and dykes. Such more acid rocks as andesites occur onl}’ 
rarely, and still more seldom are (|uartziferous masses met with in some of 
the bosses. 
Dolerites and Basalts. — The great majority of the lava-form rocks 
connected with the puys are basic in composition, and belong to the family 
of the Dolerites and Basalts. They graduate, on the one hand, into ultra- 
basic rocks such as lindmrgite and picrite, and on the other, into compounds 
that approach andesites or trachytes in composition. A large series of 
sjiecimens from Central Scotland was studied a few years ago bj’ Dr. Hatch, 
who proposed a jietrographical classitication of the rocks, and arranged them 
in a number of types which he named after localities where they are well 
developed.^ More recently the rocks have again been subjected to micro- 
scopic investigation by my colleague Mr. Watts, who, confirming generally 
Dr. Hatch’s discriminations, has made some modifications of them. He has 
furnished me with a revised classification (p. 418), based on purely petro- 
graphical considerations. The doleiitic and basaltic series may be grouped 
into two divisions, one with, and the other without, olivine, and each division 
may be further separated into a dolerite group, which presents an ophitic or 
subophitic sti'ucture, and a basalt-group in which the groundmass is made u]i 
of felspar and granular augite, and possesses tlie “ intersertal structure ” of 
llosenbusch, or consists of idiomoiphic augite embedded in felspar substance. 
The term “ sub-ophitic ” is employed by Mr. MTitts “ to imply that the 
augite grains are neither very large nor very continuous, optically, and that 
they rarely contain entire felspai'- crystals imbedded in them, merely the 
ends of a group of these crystals as a rule penetrating into them.” 
Transitional forms occur between many of the following types by the 
increase or diminution in the relative proportions of the constituents. Thus 
it is not easy to draw a line between 2& and 2c; the latter again shades 
into ' 1<1 and 26' by the decrease of the felspar. 
Mr. M'atts has further oliserved that the rocks containing no olivine 
offer greater difhcidties in classification than those in which that mineral is 
present. “ The very distinction,” he remarks, “ between dolerites and 
basalts is les.s marked, the types are much less sharply distinguished, and 
decomposition and masking of the structure are more common. MBiile 
using the term Dolerite for such rocks as have a sub-ophitic structure, 1 
have extended it to those rocks in which evidence exists that a great part 
of the crystallization took place under intratelluric conditions. Although 
Professor Branco in tlie work already cited p. 46. Denudation in that region has bared the 
cones and exposed the structure of the necks which, down to even niinute details, repeat the 
phenomena of Carboniferous and Permian time in Scotland. 
^ This olassilication was given in niy Presidential Address to the Geological Society, 1892, 
Qicart. Junrn. Geo!. Soc. vol. xlviii. p. 129. See Report of Geological Survey for 1896. 
VOL. I 2 E 
